What is common wire in a three way when all the wires are black?

If this is a new installation then you have a choice of picking any one you wand to make the common wire. Usually a three wire cable used for three way switching has a white, red and black wire in them.

If this is an existing installation and you have misplaced which wire is the common you will have to go back to the switch to find the conductor used. Disconnect it from the switch and temporarily place it on the ground wire. Next go to the next junction box and start ringing the wires to ground. When you find which wire is grounded, this is the common conductor.