Let me echo everyone else: Angkor Wat is well worth seeing. One of the most impressive temple sites in the world, beautiful restored (and this from an anthropologist). Comparable to Teotihuacan, Tulum, the Great Pyramids, of the Great Wall.
That said, Angkor has gotten very touristy. I'd avoid the package (primarily Korean and Chinese) hotels on the road between the airport and town. The old Raffles Hotel is quite nice, if you can get a decent rate, or the Aman, if you're rich. Alternatively, the foreign correspondents club has nice, less expensive rooms....or check TripAdvisor...and do your research.
Three days is enough (barely). I'd do the Lake (forget the name) too. Food adequate, but not spectacular (Thailand is much better). Definitely hire a Tuk Tuk or car; don't need a guide (do need a guidebook), but may want.
My memory is that flights from Bangkok are kind of expensive, but definitely fly...the overland or boat trips are fairly difficult. I flew in from Manila on the Cebu Pacific non-stop last year for $200 round trip, so you might do better from other cities.
Enjoy!