You can simply divide equally among the three companies or divide the fee based on usage. Count the volume of mail received for each company. Each individual company's share would be Individual Company divided by the Total.
Surprised nobody answered this as it is a very simple question. How significant our the PO box fees to each of the three businesses? Are all three owned by the same people? I can say there are a number of acceptable ways to split them up. For example, you could split it by use, so if one business has 80% of the shipments that go through the PO box, then charge 80% to that business. Or you could split it evenly if the volume is not predictable. If they are all by the same owner, it really doesn't matter very much how you allocate it. I assume, you are concerned about this for tax purposes? Please feel free to give me a call to discuss further.