Hi! Welcome to the forum and congratulations on your gorgeous boy.
Do you mean Top Fin Water Clarifier, or Top Fin Water Conditioner? If you are just using a clarifier and not a conditioner, and you are using tap water for your tanks water, you need to go out ASAP, like now, and get a water conditioner to remove the chlorine from the water. If you don't it can poison the fish.
Back to your regularly scheduled thread.
Since you are doing a fish in cycle I'd recommend you use Seachem Prime for your water conditioner. Prime binds with ammonia and nitrite and will make them safe for fish for 24 to 48 hours. After that it releases them back into the water so do NOT skip on water changes.
Rather then a betta heater, unless it is a preset one, I recommend you get either an adjustable heater (best) or a preset one. The betta heaters generally stay on all the time and rely on the volume of water and temperature of the room to determine how warm the water gets. Hydor Theo 25 watt is an adjustable heater that is pretty reliable and it will fit in your tank.
Betta are tropical fish so aim to keep his tank 78 to 81 degrees.
They love plants live are best, silk are good, make sure any plastic plants you get are soft so that they do not snag his fins. If you want an easy to care for live plant try and anubia, they are very hardy and forgiving.
Yes, your betta is a male, but I'm not sure about his tale type. He'll happily live alone. If you want tank mates for him you'll need to get him a bigger tank, at most in a 3 gallon I'd only get a snail and then only once the tank is cycled. As far as females go, it's tricky to keep them together and while it may work it has a high failure rate. To keep a sorority you'd need at minimum a 10 gal, densely planted, tank and 5 females.
Keep an eye on that filter, if it looks like your boy is stressed, and staying on one side of the tank away from the filter, the filter may be too strong for him and you'll have to baffle it to slow down the water flow.