On the off chance that you prize both Gengars when you attempt to "See Off", you deserve to lose. This just proves that the list obviously isn't as tight as it should be, you have several spaces floating around. Do you know what I would do for even ONE free space in my Nats deck?
...
What I mean is..
You are weakening your deck by adding a "Dead Card" in 99 out of 100 games you will play. Sure, on that 100th game, you'll be glad you played the 3rd Gengar. But is it really worth those other 99 games when you could have had a free space for anything you'd like?
I would literally KILL for a free space right now.
Keep in mind that See Off requires that the card you send to the Lost Zone originate from your
deck. You start the game with 7 cards in your hand and 6 in your prizes. On your very first turn you are required to draw a card. That's 14 deck slots.
Assuming that you do
not mulligan, the probability of having no Gengar (Prime) in your deck at the beginning of turn 1 after you draw is...
24.14% if you play 1 Gengar
5.14% if you play 2 Gengar
1.06% if you play 3 Gengar
0.21% if you play 4 Gengar
Obviously, you can use Pokemon Communication, PONT, or even Judge to put Gengar(s) from your starting hand back into your deck, so the actual probability of being unable to See Off a Gengar altogether on turn 1 is going to be lower. From my experience, Mewgar builds without Mime Jr. do not suffer from "dead draws" by playing a 3rd Gengar. That's not to say that I'd put in 3rd Gengar in my build.
Playing a 3rd Gengar, in my opinion, is a style choice. It's merely a trade-off between the consistency of Seeing Off Gengar turn 1 and having the occasional dead draw.