The pros and cons of each system? You think the US would only use 3 types if there was any strategic value to more? We have lobbyists and congressmen salivating at the whisper of evidence that more war machines need to be built.
They exist because they are old outdated technology that the host countries feel no pressure to replace since they have to buy their replacements not build them and thr expectation of war has been marginal for a long time.
Even if they were each best at something, diversity comes at a very high cost. Every additional design of MBT means an additional supply chain for parts and ammunition. Additional personnel are needed for every line of MBT.
Since you have a different logistics line for each tank, they can't support each other on the battlefield without bringing their whole logistics with them.
They exist because they are old outdated technology that the host countries feel no pressure to replace since they have to buy their replacements not build them and thr expectation of war has been marginal for a long time.