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Okay I primarily play 4v4 so it should be fine in 3v3.
Vickers MG is solid if it can get to vet 2. At vet 2 is gets a suppression bonus and is vital for its late game viability. Luckily its pretty easy to get to vet 2 since its high damage gives it lots of vet. The hardest thing about the Vickers imo is that you have to keep it alive with vet for it to be good, considering the tools all factions have late for wiping squads that can be difficult at times.
The UC will typically die once a LV comes out. If not its fine to suicide it if you need pop for something more important.
If 2 commandos is your thing then by all means go ahead with it. Just know to limit the amount of other squads you have. I would recommend going to 1 RE instead of 2 then and max 3 IS.
The valentine might come out slightly later than the AEC since there is higher resource income in big team games. However it should be able to deal with all LVs except a puma easily.
I usually only go one. 2 is great if you plan on going centaur rush and skip AEC and need the AT. In other situations 1 at gun + aec is better.
The AA on the centaur is fantastic. Typically it will destroy a plane in a single burst. The centaur is great against retreating squads also.
The centaur is solid if you have proper AT fielded. At Guns and a AEC can ward off mediums decently, while mine can prevent diving. Also in 3v3-4v4 typically you can rely on a teammate getting a medium tank or TD early.