From Philippians, Colossians, Philemon by Dennis Hamm, SJ, commenting on Philippians 3:17:
Paul’s exhortation to be imitators of me may sound strange to our ears. We are reluctant to present ourselves as moral or religious examples. But in the ancient Greco-Roman world it was a common and acceptable practice for teachers to point to themselves as examples. It is human nature to look for examples to imitate; and teachers know that their role inevitably makes them examples, for better or for worse.
Moreover, we acknowledge this fact in plenty of ways in our own culture: consider the recovering alcoholics who tell their stories to other alcoholics to encourage them, or excercise enthusiasts who speak of the new energy they have gained from an exercise program in order to prompt sluggish friends to join them in the gym.
Paul describes the imitation in a careful way: join with others in being imitators of me, literally, “be co-imitators of me.” Since this word appears nowhere else in the New Testament, nor in Greek literature generally, it seems that Paul has coined it to emphasize imitation as a communal enterprise; they are to collaborate as a church in emulating Paul’s way of life. Indeed, he develops that idea: observe those who thus conduct themselves according to the model you have in us. They are to imitate one another as they follow Paul’s way of imitating
Christ himself. The plural “us” underscores the fact that Paul is not the only model. For example, his cosender Timothy and the Philippians’ emissary Epaphroditus are also to be viewed as models (2:19–30).