I highly recommend cycling the 3g, it's better for the stock. Small tanks will take a stable cycle, especially if you plant it heavily as a backup. A sponge filter will take care of your bacteria colony housing and is gentle on the fish.
How much ammonia is in your sourcewater? A cycled tank will remove that, giving your fish less toxic water than what you drink. Use Prime during water changes, or whenever inn doubt, to convert the ammonia to harmless ammonium, which is good for nitrifying bacteria and plants.
Performing a fishless cycle in your 3g will free up the 10g should you decide to stock it. Run both filters in the 3g. This will develop bacteria colonies in both filters. You can then install one of them in the 10g for an instant cycle. If your 5g's are not yet cycled, you can put a couple more filters into the fishless cycle. When a filter is cycled, the tank is cycled.
Bleach is best for serious tank cleaning. Just make sure you rinse thoroughly and air-dry completely.
Last edited by Hallyx; 12-09-2012 at 12:44 AM.