Placing one player in all three positions of the depth chart makes it likely that he will get 48 minutes, but does not guarantee it.
Aside from blowouts, injuries, ejections and tiredness (low stamina), the coach can substitute a player for other reasons if he's trying to win a game: for better free throws, rebounding, or three-point shooting.
As a general note, getting training minutes is intentionally made challenging.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."