0 ammonia, 0 nitrites, 0 nitrates

I set up my 3.5 gallon tank with its filter, heater, and conditioned water (prime) on Wednesday evening. I added a bit of that topfin bacteria starter. I put the betta in the following evening, Thursday.

I did a 25% water change (not cleaning gravel) this past Saturday.
It's now the Monday after and I finally tested my waters and got zero on everything.

He swims a lot, is active, responds to me, flares at times, eats well.

I'm disappointed to say I prepared everything except for cycling, which I only found out about when I put the betta in already Advice?

(named after author, Kazuo Ishiguro, as I got him the day he won the Nobel Prize in Literature)

What are you using to test the water - strips, liquid testers or tablet testers?

I am using the API freshwater master test kit.

That tester is the one I use.

Just to check, do you shake nitrate bottle #2 as the instructions say - or preferably more than that - as the shaking is needed to get one of the reagents back into the liquid.

It does seem a bit odd that you have no ammonia showing up. Though you do have live plants so it is possible that they are taking up the ammonia made by the betta, and with just the one fish there won't be too much ammonia anyway. And if the ammonia is being used by the plants you won't see any nitrite or nitrate higher than that in your tap water.
If you do see any ammonia, more plants would be the way to go, especially floating plants. But if they don't remove the ammonia and then you get nitrite showing up, you will need to do water changes as often as necessary to remove ammonia and nitrite.

Welcome to TFF.

I concur completely with essjay.