How do I aportion overheads cost to a project
I produce e.g 400 units item A, 600 item B and 1000 item C per month
The total overhead costs are R10,000.00 per month
Overheads break down is
Rent - R5000
Indirect Labour - R3000
Other - R2000

It would be very reasonable to apportion
The total overhead in proportion to the number of units.

How do I find the multiplyer for the mark up on each job to cover only overhead expenses?

Do you mean to break even? If so, once you've calculated the allocation of overheads to the 3 items then you should easily be able to work out how much you need to sell a unit of each item for to cover the overhead costs.