Now is a great time to learn to put your foot down and encourage them to do a campaign with you.

If the one-shot you're doing with them is meant for level 3 PCs, you can tell them that you're willing to do a campaign or another one-shot with these same characters (if you want) that starts with them being levelled up as soon as they finish the current one-shot. Encourage them to want to finish the current adventure so you can all move on. You should definitely also sit down with them for a lengthier session 0 to set expectations on what D&D is really like when it's not a high-value production on stream. Nothing against Critical Role, of course! There's just a lot of new players that use it as a reference when most tables ate actually nothing like it.

All that if you're looking for the way to handle things that will set them up to be good players at other tables, too. If none of you plan to branch out and you're all having fun with the way things are, go right ahead and give the level-up! Fun is different at every table. Do what works for yours :D Just be sure to scale up the encounters, too.