Hey guys, new to this site but loving it already. I have a couple questions about eating to gain muscle.

I'm 5'8", 150. My body is fine, I don't want to gain weight just to gain weight, but I definitely want to pack on some muscle. I'm slightly toned, but I want to be very well defined...not body-builder status, but definitely noticeable muscle. I have a couple questions...

1) How much weight can I pack on in 3 months? A year? I don't want to have unrealistic expectations.
2) How can I eat to maximize muscle but minimize fat? I'm always scared that if I eat too much it will just all be fat.
3) I'm beginning to train for a Triathlon next June...will this intense amount of cardio make it impossible to gain muscle?