Strategy Informer  

Or perhaps the other Valve franchise where the next installment is number 3, L4D?

Or all three?

Lets go down the list of games with a 3rd game possible:
Half Life 3
Half Life 2: Episode 3
Portal 3
Left 4 Dead 3
Team Fortress 3
Or maybe a new game somehow incorporating 3 in it...

I seriously hope it's one of the Half Lifes' or portal.