If it only requires a ratio of of 3:1 to storm a castle why didn’t the Freys just throw 600 hundred men (preferably from other houses) at Riverrun and take it before jaime gets there?

It takes 3:1 to win on a standard battlefield assault. Castles add an enormous defensive multiplier, which is why the average garrison was usually tiny (100 men or less). As a result, if you look at a lot of famous sieges, the garrisons were usually incredibly outnumbered by the attackers.