The way I run it, is group A will get run more than B, and C, for obvious reasons. They can, because they can do it.
How I get group B (for example, same as C) to get up closer to group A is the TYPE of workout. If they aren't complaining of any issues, I will have them do (for example) speed workouts (track workouts) or tempos higher than they typically do. But err on the side of safety not to hurt anyone. Same with group C.
As soon as they start complaining, I move down a notch (and I mean a notch, not completely let them off). I typically do this by adjusting their mile pace times or if I'm doing a track workout with them, their 400 times, by X number of laps.
Now if it's just a single person, I can move them within the groups accordingly. So if someone from group A tells me straight up they can't do group A's workout, I will move them to B. Likewise, if the athlete and I see it is possible to move from group B to A, I will move them, and closely monitor their performance and what they tell me.
Again, what helps is having someone there with them. I typically go out with the B and C groups and run with them, and do the workout with them. This way, they know EXACTLY what they are doing. Basically someone with authority over the workout is there with them. They appreciate it too!
The A, B, and C groups will all get a great workout in, and they all improve drastically from the start to finish of the season, barring any fatal injuries (eg. stupidity/showoffs).
Been doing this for a while, and with 98% success, minimal injuries. This is my FIRST semester in 10 years where 30 runners successfully completed all races without injury with all of them agreeing upon positive success. I must be doing something correct with my coaching career.
A lot of it helps that I run with them, and I instill communication. I hear a lot when we go out for 8-10 miles. A lot of people's inner feelings come out, especially when their teammates are around. Sometimes they joke with me, ahh, the coach has to hear this.... but I find all of it interesting. If I need to, I put my $0.02 in.
The amount of time I spend on my program is 110%. I don't do any less. Also, if I see something, I act upon it immediately.
I enjoy doing it, and wouldn't trade it for anything else. I love coaching and they love me. We are a tight knit family.
Like I said, there are a lot of other teams that they wish their coach would go run with them. I guess it's far and few between nowadays.