typical efficiency of over 90%
So I'd use 80% as a more conservative approximation. 20% of 3W would be 600mW so yeah, that thing will likely get intensely hot in your case. Even at >90% the device will heat up pretty badly since most of the dissipation will in fact happen in the IC, esp its built in mosfet.
You're asking 1A from a very tiny package. We've come a long way, but magic still is hard to come by.