Leveling out audio from multiple tracks?

Today I recorded a podcast with three people with three separate mics. The guest has never spoken into a mic before and definitely not for a long time like he did today. Anyways, his audio is all over the place and I didn’t like to keep telling him to get closer or farther from the mic. So…is there some sort of program that can auto level the volume out? Turn it up and down as needed? It would be great to know for the future also. I know I can manually do it and I have done it before but not for 4 hours of audio. I’m using GarageBand if that makes any difference. If anybody has any tips (other than recording properly in the first place lol) I would be greatly appreciative.