So let's say in a match-up between Phoenix and Sabretooth, Sabretooth hits Phoenix for 3 damage.

Given Sabretooth's attack power that reads:

MORTAL WOUND: When a character takes 3 or more damage from Sabretooth's attack, that character can't heal for the rest of the game even if this power is countered or lost.

...does it counter both of Phoenix's powers?

PHOENIX RESURRECTION: Phoenix can use Support. After actions resolve, heal her of damage healed by her use of Support.
THE PHOENIX REBORN: Once per game, when Phoenix would be KO'd, instead heal her to click number 8.

I can see it countering her 'Phoenix Resurrection' power, but what about the 'Phoenix Reborn' trait?

And does the 3 damage from 'Mortal Wound' need to be from a single attack, or is it a cumulative thing? So, 1 on the first attack, 2 on a second attack?

--
bS

It doesn't actually counter at all. (Counter is a game term.) But it will prevent Phoenix from healing.

The trait is healing so she will not be able to use it. As for the special power, she can use Support on others, but she cannot heal herself.

It needs to be from a single attack. Also, if he has a 3 damage value, but it is reduced by Toughness, etc. then that is insufficent. Phoenix needs to take 3 clicks of damage.

interesting, i also wonder if it would stop her from going back to her 8th click after KOed?

It would prevent that. She needs to heal to do that and she cannot heal.

From the Player's Guide:

yep. i did this exact thing to phoenix while she was on her 16DV with regen (ie: 8th click). she took all 3 of that glorious damage and then took a dirt nap
Sabretooth is a killing macheen meng!

What is it with villains getting powers to shut down their heroic enemies? First Bullseye and now Sabertooth? Madness I say!

Kid Zoom shuts down almost everybody, not just villains