For aquatic turtles, I suggest having no substrate. The turtle rugs you see in the stores are for terrestrial turtles and tortoises and would just float around in water. I use the rugs for my sailfin lizard. Some people put aquarium gravel in aquatic turtle tanks. I think it makes it so much harder to clean. All the uneaten food falls into the gravel and rots. Plus, the turtles sometimes eat the gravel.
I have yet to find a decent filter for adult turtles so I made my own! For baby turtles, I suggest Duetto filters as they can lay on their side and filter only a few inches of water for hatchlings. Once the turtle is over about four inches, the Duetto won't be enough filtration even if you clean it daily. I think the filter at DFS to which you're referring is the one which is a hang-on canister filter with rain as an output. If you look at the gph, it's only 79 gph. The Duetto 100 which I used in baby turtle tanks is 100 gph. So, it's even weaker than the Duetto even though it looks larger. That filter will work fine for baby turtles but not adults. Since you have not one but three young adult RES's, you'll need either a stronger filter or more frequent cleaning. To give you an idea, I have a single 6.5 inch turtle in a 40 gallon breeder tank. My pump is 350 gph (more than four times the flow as the filter you found). I do a weekly 100% water change at which point the water is plenty dirty!
The 40 gallon breeder has a good surface area versus price. It's good for turtles up to about half a foot long. It's getting too small for my single turtle. Since you have three turtles, ideally, you would have more than 100 gallons. There is a 120 gallon glass tank but it's expensive. Once you're over 100 gallons, it's most cost efficient to make an indoor (or outdoor in warm weather) pond.
The upturned shell of your one turtle may indicate cramped quarters, an improper diet, and/or improper lighting.
I'm not sure of where you can buy snapping turtles (hatchling or adults) in Canada. A great page on snapper turtles is at:
http://www.chelydra.org/I live in Maryland, USA, just south of Pennsylvania. Both states have native common snapping turtles.