GREAT question & some of it has already been answered with the "how big is your gear" & "How many ways/times do you want to split it" & then there's the "How much do you want to experiment?" question....
Personally, I used to use a 3-tier system, but on 5 levels. YES! FIVE levels. Bear with me on this...
I designed & built my original system that involved 2 HLT's (50L each), 2 boilers (80L each)& a single MLT, all gravity-fed.
The premise was to:
1. Create multiple, separate beers in a single brew-day (reclaiming heat from the chiller to heat/start the next mash & have them completely different).
2.Parti-gyle from a single mash & run-off to two separate boilers equally for a mega-batch, or (still parti-gyle);
3. Produce a heavy from one boiler & run the lower-gravity runnings into the second boiler for a "light".
I think I've echoed previous posters' questions:
"What do you want to achieve?".