hi,
well, it is always possible to combine 900 1800 and 2100 onto one antenna system (using a triplexer 3 -> 1 and a triband antenna)
The main problem is going to be combining the operators using the same band. That requires using a combiner (-4dB loss for 2 inputs (=2 operators 1800 in your case), -5dB loss for 3 inputs (= 3 operators 2100 in your case) )
depending on how many floors you must cover, the biggest choice is to use “analog” transmissions, or “optical” transmissions (or digital).
If analog : you’l probably need some amplifiers on the way, because feeder losses are gonna take their toll !)
If optical : you’ll need A/D and D/A convertors. It is the most expensive solution, obviously.
Visit RFS website, they can give you some good ideas, i suppose.