My wife sells filters at the wholesale level, and agrees that the cheap throwaways are your best bet. Washable filters don't filter well at all, they only catch LARGE particles, like bugs, lol.
If you can find a filter warehouse, if you're really nice and bring dougnuts they'll sell you a dozen for ~$50
I'm surprised you can find 3-packs that cheap?? They must be crap!
-edit-
Browsed Homedepot.com, I see the one you're talking about... WOW you guys get cheaper stuff in the US! Here in Canada the cheapest 16x20 filter is $16 at Home Depot!