Yes, you are right that 3.1 is the front three speakers plus a subwoofer, with no surround speakers. I don't think anyone makes a 3.1 receiver. So you need to buy a 5.1 receiver (or more channels; all of the better ones will have more channels). In the setup process, you will tell the receiver that you will not be hooking up anything to the surround channels. I suggest downloading the owner's manual for whatever you are considering buying and read it carefully before you buy, to make sure that it has all the features that you require. And it will probably be confusing reading, no matter what model you select.
If you are not wanting fancy features, you don't need to spend that much on a receiver. And if the specifications listed for the Hsu Ultra 15 speakers are accurate, you don't need a lot of power to play them fairly loud. And if you do want lots of power, buying a receiver with preąmp outputs for all channels and a separate ąmplifier(s) would be the way to do it. But you probably won't need it with those speakers for sane volumes.
I personally would look at the lower end of Yamaha's Aventage line of receivers. Like this one:
RX-A830 - AVENTAGE - Audio & Visual - Products - Yamaha United States
If that one does not have all the features you require, look to a higher model, but otherwise, I would recommend buying that one and saving some money.
Yamaha tends to give good performance at a good price with superb reliability (if not abused).
Basically, speakers are, by far, the most important thing affecting the sound. So that is where you should be putting your money if you want the best sound for your investment.
You will also want to carefully follow the directions in the manual of the receiver for proper setup.
