Hi Gidget and welcome to TFF!
For 3 weeks in to an Add&Wait Fishless Cycle, you sound like things are going ok. The process is not linear and the feedback from the two different substances, ammonia and nitrite(NO2) are not necessarily linked to each other. And just because one is able to drop to a low point one day doesn't mean it will necessarily maintain that low point from then on through the process, so ammonia could hit zero but bounce back to 1 even though you are measuring at the same number of hours from the time you added ammonia.
One thing often confused in the add and wait method is that you only ever add ammonia at the 24-hour mark from adding it previously, even if it dropped to zero earlier. And you only add of course if you dropped to zero withing that previous 24 hours. Typically at this stage you'll want to be testing twice a day to heve an idea what's happening at both the 12 hour mark and the 24 hour mark and you'll want to be recording those in your aquarium log so that you can look back and see the trend. Trends are more important than a measurement on any given day since that's just more or less a random snapshot, given all the things involved.
rolex's comment is also right, that the nitrite(NO2) test can sometimes give a confusing result during the largest nitrite levels, turning a strange blue-green sometimes when the drops hit the water and there's a lot of nitrite(NO2) in the water.
The goal in a fishless cycle is for you to put 5ppm of ammonia in and then find 12 hours or less later that ammonia has dropped to zero ppm and nitrite(NO2) has dropped to zero ppm. At that point you can start your "qualifying week" and keep watching it perform that feat day after day for a week, at which point you can be very confident that it won't "mini-cycle" on you when you get your first fish stocking.
~~waterdrop~~
