Context tells us whether the certainty (a) is about the present, as in the example above, or the future: Prince Charles will become King when the present Queen dies.
Habitual characteristic (b) is something that is shown regularly. It was observed in the past, can be observed in the present, and will presumably be observed in the future. It is not possible to know of a future habitual characteristic, so we do not use this form for purely future reference.
Volition (c), denoting an offer, can refer only to a future situation. Will is normally contracted to ‘ll.