If you only had fish in your tank for about a week, then your tank has not went through the cycle yet. Doing Nitrogen cycle with fish usually takes 3-5 weeks unless you are seeding your tank with media from someone else.
The amount of fish will depend on how large your tank is as well, 3 fish might be good for 10-20gal but will not be sufficient to start the cycle on a 55-70gal tank for example.
Your tank should of went through bacteria bloom the day when you put the fish in. Bacteria bloom usually results in very cloudy water (white cloudy, not milky or yellowish) After that you should of seen your ammonia levels spike up in which you should of done a PWC
to bring down the ammonia levels. If none of that has happened, your cycle hasn't begun yet and needs your attention