Which one is correct:
estimated as
estimated to be
estimated at

I am fairly certain 'estimated at' is incorrect. What about the others?

I think it depends on the context...

I believe "estimate as" is usually, if not always incorrect

"estimated at" MAY be correct, depending on context, i.e. His net worth is estimated at between $15 and $16 MM.

"estimated to be" MAY be correct - That Egyptian period is estimated to be nearly 3000 years old.

For the GMAT, estimate to be is usually the correct phrasing...

On the GMAT, always choose estimated to be, even with your "net worth" example!

-dan