Personally when using poseidon i like making stables and academies once i hit heroic(stables and archery ranges on classical). Hippikon can counter archers. Prodromos can counter cavalry. Hypaspists can counter infantry and hoplites can counter anti-cavalry cavalry(prodromos/camels). So stable+academy combo beats all units on aom. The bad thing is that you dont have ranged units so if you get big army, pathing becomes a problem but still i think you can beat any possible combination even ranged units. This combo has a problem against extremely massed tas.
If you arent poseidon then it is suggested to make archery+academy combo. This combo beats pretty much everything but might have a problem against mixed tas+jarls. The major disadvantage for this combo is that you cant counter raids and neither you can raid that effectively cause hoplites arent that fast. Other than this raiding disadvantage, i think its the best combo(especially if you choose the right minor gods). Especially when it comes to pop effectiveness this combo rulez.
If you make archery ranges+stables combo then huskarls will eat you alive so this isnt suggested.
Of course you can always throw all 3 military buildings but this means tons of wood wasted and most important, you wont have enough space to put all those buildings. Not to mention that its easier to concetrate on 2 buildings cause you will only need to buy upgrades for those units.