While I believe that followers definitely should NOT be relevant throughout most of the game - for numerous reasons expounded upon here and elsewhere - it does seem a shame to miss out on these guys for the remainder of your playtime after you've finished Normal.
The only thing I could come up with in about 30 seconds is to make them completely unattainable as followers in Nightmare and Hell. Now now, hear me out! What would happen instead is that these three would become part of the Random Encounter system for NM/Hell only. By this I mean that you could randomly meet them in the world and they would function as the Hireling system was going to, temporary dudes that fight alongside you for a little while until you've completed an objective.
With this you're not making single-player people miss out on anything, and multi-player people get to experience the wonders of these fully voiced lore-excreters. It's a win-win!
I can see how this might be confusing, but if they still travel with your little caravan and you can still customize them however you want, it wouldn't be a complete waste of the system in the higher difficulties.