Sure you can describe a three state system with ordinary quantum mechanics. Depending on the system, you may invoke decoherence, if that is appropriate.
I can define a system that may be in state |a>, |b> or |c> and do the usual things you can do with quantum mechanics. No QFT required. You also wouldn't necessarily need to invoke schrodingers cat, decoherence or whatever either. I'm not sure what the point of that would be for this project, but there are plenty of interesting three level systems out there. The classic three level laser system that every undergrad studies would be a good example.
However, any as bhobba rightly points out, colour charge is the domain of QFT. Or more accurately, QCD.
You can motivate the requirement for colour charge fairly easily though - just consider the elementary particle Δ++, which is made up of three up quarks. Clearly, without colour charge, you would violate the pauli exclusion principle - three up particles would all have the same spin. Thus, the quarks need to be different by some quantum number, thus the introduction of "colour charge".