It is possible but you'd probably be looking at a very specialist policy. Most COMMERCIAL insurers do Minifleet policies which are usually for between 3-12 vehicles but you will lose all rights to your NCB as it will be used to calculate the best possible price.
The main issue you will have is modifications as with a standard fleet policy you will pay through the nose if you're offered cover at all as you're not a business. About the only way you're likely to get cover is to go to a specialist broker and find out how much it would be to get a private multicar policy which allows modifications. I don't think that would be any cheaper than separate policies for the 3 vehicles.
