Can I run this adventure with only two or three characters or should I make a DM character to fill it in a little? I’ve played a bit in the past but everyone else is a beginner.

You can run it with 3, but you’ll need to adjust the combat a little. Drop a quarter of the bad guys and be a bit friendly with healing options. Resist the temptation to create a dm pc. Particularly with new players that are trying to find their way in the game. While they’re fumbling away, learning things, your pc will be accomplishing al the stuff your players should be.

Look into the rules for "Sidekicks" and consider letting the players control a simplified support character. (TCoE and Essentials kit, I think)

This will both help balance encounters and ensure that your players have access to more rounded abilities. (Healing is important, a rogue is valuable, etc...)

While a nobleman/woman might have a squire, or the caravan might have another Sellsword, the "Sidekick" could also be something like a bribed goblin, or tamed wolf from an early encounter. (Of course, a wolf isn't likely to be able to fill the healer role)

You can do it. Just adjust the combat to an appropriate difficulty for the number of players.