I've been through several acquisitions and a few IPOs. If the company is acquired, you should contact the CEO and the CFO to ask the price per share at which the company was acquired. If you have vested options, they should send you a letter letting you know whether your options are "in the mon...
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.