When beginning a new campaign in 5e, I often hear DMs and players suggest it’s best to start the characters at 3rd level. I suggest a different path: begin at 1st level, but let the characters advance to 3rd level quickly. This assumes you’re using milestone levelling, which is my personal preference. Also note that I don’t believe you need to let players advance at a constant rate. I believe that the higher the level the longer it usually takes to get there.
If you’ve got new players in your group they learn from everything they play through, so it’s better to have them experience 1st and 2nd level even if for a short time. It gives the new players a chance to get used to roleplaying a character and how combat and skills work during the first few levels, before they have to make choices of what path their characters will follow, and learn to use stronger abilities.
Even if you’re dealing with experienced players you’ll find that playing through the first few levels gives the players time to get used to their characters and work out how to act as a group. This usually doesn’t take that long, so after one or two sessions the characters can advance to third level.
Your players will want to be at third level right away because it gives them more power but there is a value in those first couple of levels. You can compromise by advancing the characters very quickly so that they will get that power soon, while still allowing for the value of the first few levels.
