A quarantine tank can be any size you want it to be. I have a ten gallon that started out as a quarantine but slowly it has evolved into a planted guppy tank. Generally speaking, a quarantine tank is supposed to be bare bones so that you can isolate sick fish or plants and use medications if needed. 3 gal is definitely too small for a quarantine tank though. Its just too difficult to meter medications in a small tank like that because it doesn't leave much room for error. Most medications are designed to be added to a tank in 10 gal increments, usually 1 dose for 10gals,2for20 etc... Id say 10 gals is just right for a quarantine tank.
75 gal,pressurized CO2 with controller, 3WPG PC lighting,gravel/flourite 50/50 substrate, Filstar XP3, GH=7,KH=5,PH=6.9