If you mean that employees are employed by company A but sometimes do work for companies B & C, then I would raise a sales invoice in A and post it to Other Income.
Then in B or C's books I would post the invoice to either direct costs or expenses depending on the nature of the work done by the companies.
It probably help if you post a little more info on what the 3 companies do?
-- Edited by ADAS on Sunday 18th of March 2012 11:48:00 AM
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Tony
Responses are intended as outline only. Formal advice should be sort from your Institutes Technical Department or a suitably qualified Accountant.