basically three lvl ups of a merc are same as 2 lvl ups of a normal char..
so after every three lvl ups just distribute the points considering 2 lvlups for a char..
ex.. a merc got twlve points after 3 lvl ups... if u making crossbow merc, then distribute str n dex as 8, 4.. just as u wud do in a cross bow archer..
If u lvl up ur mer as sword or bow raise the stats str 2 and dex 2 every lvl up and if u r lvling up ur mer as ele mage raise the stats as int 3 and dex 1 every lvl up and if u r lvling up as xbow,axe,mace,spear,non-ele do as shrikant128 said.
All the best