If you mean white/blue/green then you'll need between 3 and 4 volts for normal 5/3mm ones. I think the specs say 3.2 or 3.4 volts max, but they do vary by individual. The red/yellow/orange ones run at a lower voltage. 2.something usually.
Otherwise the other info here is correct.
Some folks will also warn you that the forward voltage on different LED's is different enough that if you wire them all in parallel that if one has a low enough voltage it will run away and overheat while the others are OK. I have not had this problem with LED's that I bought in a single batch, but definitely see it between purchases. You can run at a slightly higher voltage and put a separate resister on each one.
If it's possible to get enough voltage to run several in series, this is the way I prefer to do it. They even out quite nicely that way.