if he trained the sannin when he [sandaime] was hokage, how did they do missions out of the village..? i doubt the hokage would go for a mission such as a escort to another village..
i know he was a VERY strong ninja, but he still has his hokage duties and being away for a week or two...i dunno, i could be wrong..

EDIT:
besides, usually the hokage choose the next heir among the students THEY train
isnt it funny the only one who wasnt offered the Hokage title was the one who wanted it..
Orochimaru!