I would win. I'd just wait til they all killed each other.

If there is only one survivor there is only one story.

Most likely they'll level a planet before each one dies, and if your on that planet... You would die in the conflict.

which leaves us with my suggestion... take off and nuke the entire planet from orbit

You do realize you will need an entire fleet to destroy the planet?

Even if just one of those 3 were on the planet, the imperium would do it.

Gazghkull was on a planet, Armageddon. They didn't nuke him then. Also when Fulgrim (when he was still loyal to the emperor) discovered Eldrid, he didn't nuke him, instead he tried to fight him planetside in brutal melee.

depends on the size of the planet.... and the size of your nuke

besides we just pick up the batphone and call in the inquisition... they like a good exterminatus once in a while

i guess it depends on the importance of the planet...

Inquisition fleet and ultramarine vs Ork WAAAGH fleet, Eldar Craftworlds, and the 13th Black Crusade fleet.
Yeah... good luck with that...

you misunderstand completly we wait until they have spend all their strength fighting each other... there isnt much planet left after that anyway