Your question brings up two or three more relevant area's of law.
First, the contract under which you are participating in the competitions may prohibit or otherwise limit your ability to use your work in your portfolio. You'd have to review this contract (which may be in the form of "Terms and Conditions") to determine its impact, if any, on your situation. Likewise for any contract between you and the recipient of your work.
Second, if the firms are using your work as a trademark, then you'd have to be careful in how you use the work. Any use that implied endorsement by the firm or caused confusion in consumers might be problematic.
Finally, you may or may not be the author for copyright purposes, depending again the presence or absence of certain contract terms, and so that may impact your ability to use the work in your portfolio. Same goes for the license granted to the firm, assuming you are the copyright author.