It is normal to fight at this age.
I think it is likely that he feels outnumbered, and frustrated, as you say, and that is perfectly understandable. His two elder brothers probably make him feel excluded, whether deliberately or not, and I expect they can do things that he can't do - physically, and intellectually.
You are doing absolutely the right thing by trying to build his confidence, letting have one to one time with you, inviting friends etc.
The thing is, at 9, even if you do ten positive things with him in a day, he will fixate on the one negative thing that happens.
Would it help, do you think, to have a talk with the twins and try to make them see things from his point of view? Unfortunately they are entering their teens, which is a time when they do find it difficult to empathise - although you say they are patient with him.
It is really hard being the little brother.
Also, it is worth making sure that he is getting enough sleep, and food, and not wearing himself out keeping up with the big ones.
My boys used to fight constantly until they were about 15 and 17. Now they are the best of friends.
I think it is likely that, given the particularly close bond between twins, ds3 will always feel a bit of an outsider, and only time and maturity will help.