In New Vegas, they say the Legion controlled all land east of the Hoover dam. But if thats true, why wasn't the legion in (or even mentioned in) Fallout 3? All anyone seemed concerned about in Fallout 3 was the super mutants and the Enclave.

I don't think they control all of the land to the East. Probably an exaggeration. It's a long way from Arizona to DC, especially when your only mode of transport is your own two feet. It's not surprising that the Legion haven't even been heard of on the east coast.

Legion doesn't control DC.

I think you may have taken it a bit literal. The Legion control Arizona, much of Utah, and places east like that

This, the DC area isn't really that explored with the exeption of the Brotherhood. Most just know rumors about the place, that's it. Give it a few years though, the Legion will probably be close to there.

Arizona and Utah are about as far east as people know

This ^

The Legion would not even know of the DC area. There is thousands of miles between Arizona and the DC area and Caesar wants to go West more then he wants to go East.

They controlled Arizona and Vertically adjacent area

Better question is Where IS the Legion. See a handful at a camp here, see a handful camp there. Oh that's it?

I highly doubt the legion would move eastern more. They will probably loose 1/4 of there army moving other there if not more.

But... you don't know what is East.