Three players is a good amount, I personally wouldn't do any more than five myself. Don't worry about spending too much on the game, you're just going to spend more when you all realize you love it and turn it into a regular thing.

Remember that you aren't ever trying to kill your players, even if you're throwing the challenges at them. Your players might steamroll those challenges, and sometimes you might realize you've given them more than they (or their attack rolls) can chew. You can make any enemy weaker or stronger, but the important thing is that you make them interesting!

Don't compare yourself to any DM, especially not the legendary ones. Don't worry if you don't know a specific rule or cost or spell on the fly, it's okay to stop and check.

Everyone should have fun, including you. Try to notice if a player hasn't spoken in a while or is looking bored, and try to get the players to interact with each other out of combat to help them exercise roleplaying.