The ending ? (spoilers)

I took it that Cushing is getting a break despite trying to steal some of the money, sort of a Christmas gift from the real thief and the police guy as you call him. That's why he said "sentence for what, sir?" The voices put on a recorder was what Cushing heard on the phone.

Have just watched this and loved it. IMO it's imperative to accept the film as a kind of metaphysical morality lesson, it's essentially A Christmas Carol Part 2. If the last shot had been of Hepburn dissolving to nothing, and nobody involved having any recollection of the morning's events apart from Fordyce, it would be completely inkeeping.

To answer the questions as I see them...

The third man Hepburn mentioned was the tape recorder (he say's something like 'it took three men, myself an accomplice and a tape recorder'). He specifically states Fordyce is not the accomplice. Who the accomplice is is not clear to me as I'm not sure what plan Hepburn is referring to. If it's the plan to rob the bank, then his accomplice must be a nameless man who he has worked with. But as Hepburn's real objective seems to have been to make Fordyce see the error of his ways, I wonder if the 'accomplice' is Pearson, as he was essential (though unwittingly) to this 'real' plan. I assume the phonecall Fordyce took from his wife saying she was held hostage was actually a cleverly constructed recoding, so she's the 'voice put on recorder'.

Early on Hepburn contributed five pounds to the bank's Xmas party fund, and then demanded Fordyce reimburse him the money (so as to effectively make him contribute some of his own money to his staff's party). At the time Fordyce has only four pounds and resents handing it over. Later when he borrows the additional pound from Pearson (to repay Hepburn), Fordyce shows the audience he has learnt the lesson Hepburn sought to teach him.

I can't remember the detail around the 'sentence for what?' comment. Hepburn has deliberately incriminated Fordyce in many ways, but when he see's him break down (or 'learn his lesson') he explains to the police he is not involved in the crime. The end seems a bit rushed and isn't that clear, I suspect the exchange that leads to the 'sentence for what?' comment is basically there for our benefit, to make it clear Fordyce is no longer under suspicion.

Fordyce still thought the police were going to arrest him for being complicit in the robbery.