The crucial point about Volunteering is that there must be a period during which you actually provide some service to the community at large. For this reason, CCF can't count while you are merely being instructed. You have to do some instructing  yourself, and this doesn't occur until well into the third year.

If you aim to do six months CCF in the third year it can work out like this (though I'll need to check the details later)

So you could start at the beginning and be through 3 weeks into Cloister Time, having done 20 weeks being trained and 6 weeks giving practical service. Or you could start 6 weeks into Short Half and finish at the end of the year, having been trained for 14 weeks and given practical service for 12.

If you decide to do three months CCF in your third year for Bronze Volunteering, you can't start counting it the last three weeks of Short Half. That way you'll finish at the end of Common Time, having done 9 weeks training and given 4 weeks of practical service. So you wouldn't then be able to begin Silver Volunteering until the beginning of Cloister Time, which leaves 16 weeks still to do - which will take you well into Common Time of your fourth year.