You should simply say that you are employed by Company C, which is in the process of merging with Companies A and B.
It will be important for you to clarify to what extent this merger is prompting you to seek new employment at this time. For example, is your position being eliminated, and/or are you credibly concerned that it might be? What do you anticipate that your job-description will be, post-merger? How is this affecting your decision to seek employment now?
It is also important for you to clarify whether you are the owner or an officer in any of the companies in question. Because, if you are, merger agreements ordinarily require present officers to remain in the employ of the merged companies at least for some period of time. I wouldn't want to waste my time hiring you, only to find out after-the-fact that you are contractually barred from taking the job.