From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156897-499992
Question: When starting my seeds indoors, I planted 3 seeds in each
peat pot, and they all came up. Should I transplant 2 of them?
Answer: You want to end up with one seedling per container. Yes, you
can transplant 2 of them, but you should consider whether you really
need those extra plants. Remember, if you planned your garden for 6
tomatoes, you donOt want to end up with 18 plants! A better bet is to
snip off the 2 weaker seedlings at the soil line. That way, you wonOt
disturb the roots of the one you want to keep. But if you just canOt
bear to do that, then go ahead and gently transplant them.