I'm trying to figure out what receivers can be used with which radios.

Here is what I have:

Radios:
DX3R Pro says compatible with DSM/DSM2 RX
DX3E says compatible with DSM/DSM2 RX
DX3.0 says compatible with DSM RX

RX:
x2 SR3100 DSM2
SR3520 DSM2
x2 SR300 DSM

If I'm correct all the receivers can be used with DX3R Pro and DX3E, but only the two SR300 DSM RX can be used with the DX3.0?

And if i'm correct, using a DSM receiver on a DSM2 system just makes it slower (11.1ms vs 5.5ms) right?

correct.