What are the rules anyway? Do they get ported into Marvel or DC earth after the prep time is done? Do they face the opponents in a straight forum fashion or everything passes normally as if they were in a comic book world?
I ask these because if they can use stealth, they can indeed, get advantages and try to improve their chances. In a straight fight they don't have the advantage.
I consider myself to be well versed about Kang, I've read more than half the issues he has appeared into, and know most of the tech he has displayed.
We've never seen a Kang "not-holding back" except maybe in Avengers Forever, he always plays with his food and engages combat in such a fashion that his opponent is in the ability to fight but is bested. Since I cannot really vouch for Kang "going all out" -it has never been shown so it would be speculation from my part-, I have to assume he will fight in his regular fashion. In which case I seriously doubt he could get the firepower to win this in a single year.
I think it is within the power and the resources of those three to take the two earths down, but in the time frame they would be defeated.
But again, who knows? Kang going all out could bring armies from several realities to improve his tech and wage war for him. We just don't have on panel feats to prove it could happen.