Ok, your problem is almost certainly either ammonia or nitrite poisoning.
Unless you were adding a source of ammonia to the tank during that three week period, there won't have been any food for the bacteria you need to grow in your filter. Of course the water was alright when your fish shop tested it, because it was just tap water!
Once you added fish, which produce ammonia all the time, that would have started building up in the water and poisoning them.
These toxins attach to particles in the fish's blood and gills and prevent them taking up oxygen from the water; that's why they're gasping at the surface.
You need to do a very big water change, ASAP. Try and change at least 95% of the water right now; leave just enough water for the fish to swim upright (don't forget to switch your heater/filter off first!) before refiling with warmed, dechlorinated water. Then do 50 or 60% every day until you can afford some test kits of your own. If you do get the shop to test the water for you again, get them to write down the actual numbers for you.
As TallTree has suggested, your tank (unless it's a five or six footer) is unlikely to be suitable for the silver shark. How big is your tank?