Complete Novice 3 Phase SMART CONTRACTS

Ok so I would like to lance a S.T.O. BUT in 3 Phases The Phase 1 will offer our Tokens 1 to 50,000 at 50% reduction The Phase 2 will offer our Tokens 51,000 to 100,000 at 25% reduction and Phase 3 will offer our Tokens 100,001 to 200,000 at full Token price ($10) Each PHASE would start ONLY if the precedent phase is completed ! Now would it be possible to enter all this into one smart contract OR compile 3 different contracts .. Now im a NOVICE in this so if someone could put some light on the subject ... It would help !! Thanks , David