Also, the postcode should be wherever the car is going to be kept for most of the time - if it is going to be pretty much even, then the insurer will look to ustilise the highest rated one.
Follow the advice above and get the three quotes, but make sure the car is kept at the address you state it will be for most of its time when not in use (which also should be the address of the owner) - dont be tempted to lie.
Dave
P.S. I feel obliged to point out, I am speaking from experience and my own personal views above - I am not an advisor, nor an expert.