S.P.
Here is the thing....your older son has a right to his own space, and it is up to you to see that he gets it.
Otherwise, he will resent the twins all his life.
So what if they want to intrude on his playdate.....YOU need to make sure they don't.
The family rule should be....NO fighting, period, NO name-calling...NO bullying etc.
YOU must nip any of this in the bud by addressing it the minute it starts, in no uncertain terms, and imposing consequences.
My daughter has two boys, ages 6 and 9, and she is constantly revamping rules and consequences to keep up with them.
A book that has helped her is RAISING BOYS by Dr. James Dobson.
Perhaps you could get some helpful ideas from it too.