The best thing to do is call Virgin to find out how much each cat is going to cost as they usually go on size and weight as well as how much cargo they're already carrying to the cost of transporting the pet.

Plus, you've got the customs / Pets Scheme fees to pay.
The huge cost of moving one cat over a year ago put me off, as well as our limited space for him so we left him with my mom in the states so he's got plenty of room as he's an indoor cat. For my cat, who weighed about a stone with flight & customs / Pets Scheme fees I was looking at about $1,000. So roughly, you'll be looking at about $3,000 to move just the cats, but then again those fees are bound to go up in the next couple of years due to fuel prices and normal administrative increases.

I'd budget about $4,500 at least for the 3 cats if you're planning to move in 2 years time.
Good luck!