As a member here im going to tell you that you are overstock. goldfish are messy animals so 3 in a 2x1 tank(20 gallons right?) is not going to work out well. You are looking at more like 1. (I'm not a goldfish person but i do know a little about them.)
As for you question i would recommend that for a 20 gallon goldfish tank you are looking at a filter rated for around 40-80 gallons. I would say go with an aquaclear 70.
As for water testing most people own API's master test kit. It gets the job done and is far more accuate then any strip. There are some better liquid test kits but i forgot what they were since I use API and that is all any store around here carries.
55 Gallon Community
40 Breeder Peacock cichlid Grow Out
10 gallon empty
5 gallon Guppy Tank
2.5 gallon cylinder Crayfish tank
1 gallon empty