It may be difficult going three monitors as they are usually sold as pairs until you get to the really expensive ones which are sold as singles.
You could still go with the HSU speakers as you can buy just 3 if you'd like and then get a decent little receiver. As long as your room isn't huge and you aren't going to listen at 100db+ you can get away with a new Yamaha, HK, possibly Marantz, Denon etc.
Probably have better luck finding something great used at Canuck Audio Mart for an amp. You don't need a good soundcard as you would go optical out to the receiver.
Edit: It can get a bit pricey as you would probably want a sub but something along these lines will blow away computer speakers. You could even go for a pair of speaker, a sub, inexpensive amp and then buy more speakers later if you really want them. I know Angstrom usually have b-stock subs that got for around $200 even though retail price is $800. There are a lot of options for great prices if you have a bit of patience.