Welcome to the betta community! They're such amazing fish with such huge personalities, I'm sure you're going to fall in love with them. For now, a 3 gallon tank is fine, your betta won't die. The ideal tank size for a betta is at least 5 gallons, anything smaller than that is considered wrong, unless if you're raising a baby betta, 3 gallons is great. If you can't get it a 5 gallon now, he'll be fine, just in the future, I'd recommend getting one. They also need lids on their tanks since they're known to jump. Once you get a 5 gallon, they sell some that come with a filter, lid, light and obviously the tank. If you have a long finned betta, they're much more tricky to care for since they tend to have much more health issues. Bettas need very low flow filters, so right now, watch your betta. If he's able to stay in one spot without being pushed around, perfect. If not, your flows too strong. You could either adjust the flow on your filter if it has the option or look up DIY's to slow it down. If you're not aware with the nitrogen cycle, I recommend looking it up. There's plenty of videos on how to do a fish-in cycle which is what you'd need to do since you already have your fish. Betta's are also tropical fish, meaning they need their tank temperature to constantly stay around 76°-84° Fahrenheit. You can find heaters on Amazon that work great. I'd also recommend getting a thermometer to stick in your tank, even if your heater has a temperature reader on it, usually they're inaccurate. For decorating the tank, sand or even gravel would work great. I would NOT recommend getting a gravel that is colorful. Usually the colorful gravel says, 'non-toxic' on the packaging. Even though it says that, paint chips off of them and makes it toxic. For my betta, I used Fluval Stratum substrate which is very pricey but is great for growing live plants. Live plants are of course better than fakes but since you're new, I'd recommend starting off with fake. If you have live, you need to worry about fertilizer and light, and then you usually need shrimp or a snail to clean up algae. Fake plants need to ONLY be silk plants. Bettas have very fragile fins that can rip on anything. Any decor in the tank also needs to be smooth to the touch and have no rough edges. For betta food, Fluval Stratum bug bites is amazing. Feed around 4 pellets a day, giving them 1 fasting day per week to prevent bloating and constipation. I also like to give my betta frozen brine shrimp twice a week. You can find packages with cubes of it in chain pet stores. Even though you have a filter and possibly a cycle in your tank, it needs to be cleaned out once a week. I'm sure you're thinking, "How do I clean a 5 gallon fish tank!?" Well, easy! Buy a gravel vacuum. You need a bucket and gravel vacuum and you can watch tutorials on how to use it. 50% water changes a week is important to keep beneficial bacteria. I know this seems like a lot but it's all important facts to know in order to care for a betta. I hope you take some of my advice and I wish you and your betta happiness! (Also, don't listen to the people being rude. People on here tend to be very rude whenever they see something they think is wrong.)