In my opinion knights were perhaps more important as officers than as pure heavy cav. Peasant rebelions were mostly disorganized, but you can look into Hussite revolution in Bohemia if you want to see what fighting force drawn mostly from lower classes can do if properly organized and led. There...
Of these three realms Cartage seems safest from external threats pyrrhyc kingdom is going to get hammered by celts, especially in Italy And seleukid syria Is target for parthians or persians. However Cartage is probably also weakest militarily.