How do you clean your 3 gallon fish tank?

That all depends on what is dirtying it in the first place. If you mean strip down and clean properly then any none abrasive cleaner will do the job provided you scrub all traces of the cleaner away before using the tank. If on the other hand you are refering to algae on the face of the tank then any of the glass cleaning equipment/scrapers sold in pets shops for that purpose should do the job. The new acrylics used are just about as hard as glass and should not scratch if you use sensible scrubbers.