How to lower amonia lvl in my fishtank?

you were wrongly informed... the reason goldfish need large tank is not only because of their eventual size, but also because of their HUGE waste load. even one small goldfish would be too much for a 3gallon. get at least a 10gallon tank for now. you can get a tank for $11, a filter for $14, some gravel for $3, and a gravel vacuum for $4 and you'll have a much better home for them... you will eventually need an even larger tank though.