No such thing as a 3ch receiver. Maybe some really old 4ch Dolby Pro Logic receivers from the Laserdisc era but these have no HDMI to work with modern sources and HDTVs.
Also if you won't use the surround channels you might as well ditch the Center channel. To use the Center channel you have to set the sound settings to 5.1, which means the rear channels from your source to the receiver have no speakers to go to and whatever was supposed to come out of there will just be left out completely. If you just use 2ch then all the sound that is in there comes out of the 2channels, they're just not coming out of speakers in other positions.
Depending on the sources you use you can either get whatever entry level receiver you see at a local retailer if modern HDMI protocols are important with the receiver acting as a hub. If not then you can use an older integrated amplifier that has an external connection from its preamp stage to amplifier output stage, hook up a splitter RCA plug, and then run short RCA cables from the preamp output to the amplifier output stage and then a longer run of cables to the subwoofer. Alternately, if you'll use a plain 2ch integrated amplifier, you can just get a subwoofer that has high level (speaker output) input with a pass through so you can hook up the amplifier's speaker output to run through the sub so it can get a signal from that. Either way the integrated amplifier's preamp controls the subwoofer.
