What size of main cable do i need, load is 3 x 100amp boards?? Main cable will be feeding a 4 way panel board.?

If you are in the UK then this work falls under the scope of Building Regulations Part P. It must by law be carried out by a Registered professional electrician and a copy of the Test certificate submitted to your local Authority. The fact that you have to ask the question is evidence that you are not qualified or safe to do this job yourself. If you do it then you will be breaking the law and rendering your property insurance invalid. If you are in the USA then you must check the settings on your Yahoo Answers so that you restrict your questions to USA Only. You have sent it to the "UK & Ireland Only" section. Please correct this.

Not enough info. What's the expected maximum demand? (How many machines going at once?) If there's only you in the shop, no chance that all those 100 A. boards will be anywhere near fully utilized. Do a load calculation for the shop feeder demand load. Use Watts or VA. from machine nameplates. 1 W/Sq. Ft for lighting, + any electric heat /AC, (one or the other, use largest of heating or cooling)+ your two largest machines, (or largest load running at any one time)+ any general load (small fridges etc.) + 10 KVA for 'headroom". For 3 Ph. 208V. 4W +G., take tot. VA./ 360 to get A. load req'd. In copper cable: .. in Alum. Cable: up to 60A. - #6 .... up to 50A.- #6 up to 80A. - #4 .... up to 75A - #4 up to 110A. #2 .... up to 90A. - #2 up to 150A. #1/0.. up to 120A. - #1/0 I doubt you'll need larger. Note, to meet Code, you'll need a driven ground, and a Main disconnect at that '4way' Shop distribution panel. If the 'shops' are separate structures a ground and single main disconnect for each structure.