I'm sort of in the same situation as you. Boys are 10,11,12 though and I have one and he has two. When you have 3 kids so close in age and same gender there is always, always some tension along the triangle of their relationship. It is hard when your son is an only child and then all of a sudden has to start dealing with extra kids in his space. You also have to take a real and realistic look at how your son is contributing to the dynamic. I think in my case it might be a little easier because my son age wise is the middle one. Among all 'brothers' blood or step there is an unfortunate tendency to pick on the youngest/smallest. Unfortunately for your son - that's him. 

In our case, the two brothers are not really so much alike (though of course they are in some ways). And in some ways my son's personality is between the two. My son usually plays with the older one. I had to really watch the dynamic of my son copying the bullying ways of OSS (older step son) to YSS (younger step son) and nip that. YSS is going through a phase (though it's a looong one) of being deliberately annoying to everyone around him (me, older brother, my son) except his dad. But my partner sees this. (And not criticising really - because my son is going through a period crap behaviour toward my partner - basically touching him in annoying ways - instead of giving him hugs or cuddles which is what I think he wants to do really.) 

Of course no matter what the dynamic is they will always be brothers and my son will always be an only child. 

If your stepsons have been in an abusive environment - as mine have and as my son has - then they will have POOR RELATIONSHIP MODELS and interaction styles. It's only natural. Considering both sets have parents who are emotionally and verbally abusive I think they do pretty well. 

The way we make it work is to be as unflinching as we can in our examination of our own child(ren)'s behaviour and find gentle ways to correct it. My son sometimes talks to me dreadfully, the way his father used to talk to me. We work hard to correct that. Lately OSS has started - only once - but it's obviously something he's picked up from my son. So I talked to my partner about it and when we have them for a week over Easter we will be watching the behaviour of both boys like a hawk. 

If your partner is blind to the dynamic, denies it or takes no step to address it or seems to have no sympathy with your son - then your sense of pause is right. If your partner is willing to work with you to keep a harmonious and fair home, then there is a lot of hope and you may not find a better situation. This also means examining your own behaviour - are you or your partner encouraging the dynamic in any way - babying your son (boys seem to sense this and go harder on the 'babied')? 

Another thing you can do is separate the boys with mom spending time with one of her bios and you taking the two other boys to do something fun (or just run errands, whatever). We do that a lot because my partner doesn't get a lot of 1:1 time with his kids.  Basically you need to support and encourage better behaviour and better relationships across the whole of the family you are creating - teambuilding! With both sticks and carrots. But if the other half of your management team isn't on board with this then it will not work. You cannot do this alone. 

Your partner is showing you this is allowed by not putting a stop to it. This is the beginning of your misery and it will not get better. The answer, always, is don’t marry someone with kids. No matter the question that is the answer. 

How awful for your son. Home should be the one place on this Earth where he feels safe. He shoulnd't have to endure bullying within the walls of his own home. You or your wife needs to put a stop to it. If not, please reconsider living together until the kids are older or can learn to get along. Maybe some familyl counselling will help? If not, it's up to you, his father, to protect him from this sort of thing. 

I have three. I have one, DH two. We got married when they were 11, 10, and 9. Mine is the oldest. Maybe it helped that he was the older one, more mature, and much taller. He also has a very vivid imagination and the other two need to be entertained, so it worked out that my son could invent the games and have two willing participants.

However, over the years they have formed factions. Skids against DS, YSS and DS against OSS, OSS and DS against YSS. It evened out as they aged and there wasn't any bullying or pranking.

Is your girlfriend/fiancee willing to discipline her kids? You both have to stamp it out when it happens. It will carry more weight if she does it for her two and there are actual consequences. If you do it, it will just look like your child is weak and you are coming to the rescue/picking on her kids.

I personally wouldn't start planning the wedding until you go through a few more months and see how she handles it and if things work out better between the kids.

If your son has a larger, more confident friend, I suggest bringing that kid along to things (if appropriate) so your son has a buddy, too, and someone who the other two will not be able to pick on.

The most effective way to solve a bully problem is to beat the bullies ass.  So, your son needs to bust a nose or two and end the problem.  Since there are two of them when they jump him he needs to take a baseball bat to both of his asshole SBs.

Lather, rinse, repeat.   Bullies are fundamentally cowards and one is bad enough. Two of them teaming up requires an extraordinary level of violence in response.

I was the target of bullies in my pre and early teens.  It took an epiphany of realizing that getting beat on hurts whether I was fighting back or not so if I was going to hurt, the bully was going to the hospital.  So, I sent one to the hospital for reconstructive facial surgery and year later at a new school I sent another to the hospital to get his ruptured testicles put back together.

The first incident I was jumped by two guys. The second asshole that jumped me ended up with a bisected lower lip hanging in two flaps on either side of his teeth. His buddy who was the first to jump me ran away after I hit his POS minion. 

The second incident occurred in a swimming pool when a group of 5 boys surrounded me and attempted to hold me under and punch and knee me.  As a former successful AAU and NCAA swimmer I let them put me under and proceeded to pulverize their balls as they attempted to indoctrinate the new guy into the pecking order.  All of them had to go to the Doctor but one had to have surgery. At neither school did anyone else attempt to bully me after those incidents.

So, sit your boy down and let him know that he is going to have to make a serious point with his two elder StepBros and that when he does he will not be in trouble for defending himself.

I did not initiate the fights but I for damned sure finished them in a manner that ensured that bullies would rethink targeting me. Your son needs to do the same.

Let your wife know that her sons are pushing things to the point where someone is going to get hurt and she needs to be ready for that to happen.