What does everyone do when white refuses to allow the benko with 3.d5?

3.d5 is of course theoretically the best move but some people like to play maroczy systems with 3.Nf3 cxd4 4.Nxd4. Personally I've just settled for playing a hedgehog structure but it can be hard to generate winning chances in structures like that. Lately I've been looking at the Kasparov gambit with 1.d4 Nf6 2.c4 c5 3.Nf3 cxd4 4.Nxd4 e5!? 5.Nb5 d5! 6.cxd5 Bc5

Also, some people avoid the benko altogether with 1.d4 Nf6 2.Nf3 when a benko is no longer possible. Any suggestions?

I usually allow them to take the c-pawn and usually recapture with the Q, usually after Qa5. This leads to all sorts of stuff that I usually get into more often than the Benko.

With 2.Nf3, congratulations, white cannot hope to transpose to the Taimanov variation of the Benoni. Most any Benoni defense will work great.

1. d4 Nf6 2. Nf3 c5 and then white's better off to play 3. d5 anyway. You can offer to transpose to Benko with 3...b5 or you are safe to play 3...e6 and Benoni at will. An early Nf3 blocks white's f-pawn and gives you a great game. Should white fail to eventually drop c4, they are also worse off. In fact, unlike playing a Sicilian, c3 should give you equality.

There are other ideas which are just as good, like 2...e6 or 2...d5, but given your area of opening theory interest, there is absolutely no reason not to pop c5.

I've been playing the Benko for a long time

here is how I respond

d4 Nf6 c4 c5 Nf3 cxe4 Nxe4 e5 Nb5 d5

d4 Nf6 Nf3 c5 d5 b5 (if e3 Bb7 Bxb5 Nxd5)

but what I dont know what to do against is

d4 Nf6 c4 c5 dxc5

(i know its dubious but how to exploit it?)

This is the point of the Benoni, offer the pawn for development of the bishop, much as in the QGA. Pop e6 and retake with the king's bishop.