Gravity.

Occasionally, if I'm lap-soldering wires connected to something that tries to pull them off my workbench (or a big multi-conductor wire where the connected wires pull the other about), I'll weigh one of the the wires down with a pair of wirecutters or similar tool.

I rarely, if ever use any kind of 3rd hand tool, and find them pretty much completely unnecessary. Generally, you should not really need to hold three things at once.

If you are trying to hold two wires against each other while tinning them, you are doing it wrong. Tin each wire end first, then simply hold one against the other, and heat the joint. This only requires two hands - one holding one wire end, the other the soldering iron.