Yes Damien... you're spot on - they load 3 for re-start attempts. But only one at a time. The gun/breach needs to be carefully cleaned each time too, can lead to all sorts of problems if it's not done properly.
Also, just about all the Vamps are electric starts, but the Venoms cartridge. A few Vamps (particularly single seaters) in the US were converted to cartridge though... as loads of the airfields are unattended, they can't get external power, so easier for them to use cartridges.
They are really expensive as you say... when the forces find old surplus stock the price is a bit cheaper - I believe I.R.O. £70 a cartridge, but can be upwards of £150 when in short supply.
The Hunters have a modified Vampire starter - different relays I think, but otherwise the same clockwork starter!