Suggest you do not feed anything right now as it may only add the the amonia issue.
I also suggest you follow the advice given above and peform a number of pwc
. If you have amonia present and your tank is only 3 weeks old it most likely has not completed the cycle and is not ready for fish.
You need to test daily to verify where your parameters are at and if you need to do a pwc
. If you wait until Thursday you fish could be dead by then.
Also, please describe your tank i.e. size, filtration, skimmer or not, lighting etc.
As for other livestock it may be some time. You need to first verify everything is ok with what you have.