What size cord should I use for 10 amp power tools 14/3 or 12/3 at 100'?
Without any voltage drop.

A 10 amp power tool can use a #14 cord up to say 50 feet.

Up to 100 feet, #12 would be suggested.

Voltage drop is directly proportional to distance, so, knowing the distance you have would be helpful.

Why are you using 14 or 12/3? At 100ft. And 10 Amps, 14 AWG drops 5 volts and 12 drops 3.2. Extension cords are 14 or 12/2 with a ground. Either would work but I'd go with 12 if you will be running more than 1 tool