licensing

My company is really comprised of 3 different corporations working in the same office. Although we are technically 3 separate companies, we are all sharing a network, a lot of our data, and resources. Some employees even work for more than one of the corporations. I run the IT department for everybody. I always purchased licenses for software for the total of all employees. Someone just suggested that maybe each company needs to own it's own licenses and if one company owns the licenses, it may be illegal for employees from other companies to use them. Is that true? That would be such a headache for me! I'd have to start dividing employees and setting up multiple accounts with all of our software vendors. Would you know how I can find out more about this issue?