Hi Dave, welcome to the forum.
You are only 3 weeks into having the tank set up, it is normal not to see any trace of nitrite.
Any trace of ammonia in the aquarium is no good for the fish, even though they might not show signs of it at first. Prolonged exposure to the ammonia or nitrite can cause permanent damage to the fish, it is why we do fishless cycling before getting any fish at all.
There are something you can do to minimize the damage to your fish.
1. Cut back on feeding.
Instead of feeding once a day, you can feed your fish only once every two days. The amount of food you toss in there should also be reduced. All food should be eaten within 30 seconds with no leftover at all.
Reduced feeding will reduce the ammonia produced in the system.
2. Keep up the water change daily, but do no more than 30~50% each time.
Yes, it will slow down the cycling but you don't have much of a choice since the build up of ammonia will damage and even kill the fish.
You can also use SeaChem Prime as your water conditioner to help detoxify the ammonia. By the way, what water conditioner are you using now?
3. Getting a bottle of
Tetra SafeStart.
Tetra Safestart contains both types of good bacteria needed for fishless cycling. It works as a seeder and can jump start the cycling in your aquarium.
There is one more problem I have noticed. A rather big one. It is not recommended to have guppies and betta in the same tank. Guppies do resemble betta, and betta fish are territorial. They might look peaceful now, all it takes is one incident for it suddenly to become not ok. If I were you, I wouldn't have them in the same tank.