No it's not ich.
It could be fungus. It could be the beginning of columnaris also. It could possibly be ammonia burn (though that is usually on the edges of fins). Either way, these illnesses are generally caused by water quality issues.
Your first step should be to test your water parameters. The best thing to use is a liquid API testing kit.
You already stated the ammonia is 0 (if this was not tested with the liquid kit you will have to get one because the strips and in tank devices don't read accurately). Test your nitrites (these should also read 0). Test your nitrates (these should read below 40).
If any of those readings are higher than optimum you will need to do water changes to correct them.
A 20g aquarium really is too small for 3 goldfish. Your nitrates could be high and that could be causing health issues.
I'm hesitant to recommend a treatment without knowing exactly what the illness is. There's no point in dosing blindly without knowing if it will help since meds can cause stress and some of the antibiotics can actually kill of your biological bacteria.