If two players try to steal a life from a third at the same time

[Crazy Hand 2001]

but the third player has only two lives (so can't share a life with both players; only one) who gets to take a life?

This doesn't apply to real life tournaments (aside from 3 vs 3 and 4 vs 4 matches) but it applies to all Smash games so I wasn't sure where to put this topic.

To make things more clear, let's come up with a scenario. Toon Link has decided to play against Mario, Roy and Mewtwo in a 1 vs 3 match. TL is strong enough to KO Mewtwo and Roy, and he also left Mario with two stocks left. Mewtwo and Roy both decide to steal a life from Mario, but since Mario has only two lives, he can share a life with only one of Mewtwo or Roy. If Mewtwo and Roy both hit the Share Stock button on the exact same frame, who steals a life from Mario?

Would port priority determine who wins?

 

[mario123007]

The answer is probably simple: the one who press the button first. And technically this is not called life steal, it's share stock (If your 1v3 means Toon Link vsing Mario, Roy, and Mewtwo teaming up). No matter friendly fire is on or not. In Smash4 those who press the button first will get that stock. No matter how perfect they press at the same time.
You might be curious about this. If both character gets koed at the same time in a stock match. Who wins? Well, it happened to me once. The game simply goes to sudden death mode. And our ko time on the result screen was perfectly the same. No matter you set time limit or not stock match will still count how long you survived.

 

[Crazy Hand 2001]

What if you used a Wavebird with two receivers, or the glitches in Smash 4 and Project M to control two characters at once?