To solve your specific problem, couldn´t you have them split in 3 groups at the beginning, load each group in a Marshall and then use the Join WP to have them move together?

But a Follow WP might be a nice idea anyhow.

No, I'm afraid that would not work for this. I need them to be getting out and back in several times along the mission. So I can't properly do what I want without either A) Fixing the AI to get into all vehicles in a group rather than only the first vehicle of the group, or B) Adding the Follow waypoint.

There is a 'follow' waypoint type in Arma3 - but it doesn't appear to work. You can use commandfollow or dofollow commands. Alternatively you could use a join waypoint and synch to the lead group. But first prize is getting the follow waypoint type to work.

The follow waypoint does work. They will move up after the unit they are following is a certain amount of meters away from the "following" squad.

@TakeHomeTheCup Nope they have to be grouped with the unit they are to follow. Ungrouped units do not follow. Grouping also applies to unit1 dofollow unit2 and commandfollow. Based on the grouping dependency these commands and the follow waypoint have limited application.