Best way would be to have a policy that covers multi vehicles. You would need to run it through a business policy though.
Failing that, you could insure the cheapest one and then ring back a few days later to insure the other 2. My old boss used to do that (insure his wife 1 litre and his the next day). He said they will give you a really good discount on additional cars but seperate policys though.
Deff worth ringing round as the person you speak to will get more £ if they can get all 3 vehicles. So you got a good bargaining tool