David of Broadway wrote:

Or perhaps the ticket is invalidated by multistamping, and the inspector
was being a stickler for the rules. In that case, the passenger was
riding without a valid ticket and should be subject to a fine -- but not
to /three/ fines!

That was what I meant - wasn't clear how to explain it!

It may well be the case that defacing the ticket (e.g. by stamping it
outside of the designated area) invalidates it, in which case one fine
- under no circumstances should three be due, as only one offence of
travelling without a valid ticket has been committed.

Neil