I think 3 of any gender can be great. They have something similar to one another, besides a penis.... but the boy interests. They can enjoy the same activities together, and wel... my ds3 is only 6months, but i was sitting at a bbq with a friends hubby (they have ds and expecting dd soon), and I was laughing watching the male cousins rough and tumble together, my boys were joining in too.... and my friends hubby says "That's why I wanted another boy, so ds1 had someone to rough around with".... (my friend, his wife, really wanted a girl).
As nice as it'd be to have both genders, I think the bond a trifector can have is incredible... and if I ever went back again, it wouldn't bother me so much if I got ds4... because I see the way my boys interact and love each other.... I think a 4th would be pretty amazing. Should go for a whole football team eh?
I guess a brother and a sister could have similar interests too.... but just knowing my family (I have 2 brothers), we don't share the same interests at all... but get us together with a few drinks, we'd pee ourselves laughing all night.
(My step mum has 3 daughters and 1 son, and they have an amazing strong bond, but admittedly the boy is left out a bit, although he doesnt mind, in fact i think he prefers it, cos the girls talk about their lady stuff... and well he's in the army and all manly macho and stuff lol).