Hi, I'd like to help you with your question.
You can approach her in a non threatening manner and let her know that you are concerned about her well being and want to talk with her. You may want to hold this meeting in a neutral place or even at her home, though neutral is better. You don't want her to feel ganged up on or threatened in any way. You may also want to consider talking with her yourself. All of you talking to her at once may trigger defensiveness in your daughter and she may shut down.
Start off by saying that you noticed she is following a pattern with dating married men. Let her respond to this statement. Depending on how she responds, you may need to back off and try again later or keep going. If you keep talking with her, then say that you care very much about her and that you want to help. Tell her you feel this is harming her and you do not want to see her hurt. Then let her talk.
Do not list why any of these men are bad. If you talk about the men she dates, she may become defensive and then not listen to anything you are saying. Stick to how much you care about her, what a wonderful daughter she is and how you want her not to be hurt.
If it goes well, you may want to see if she is willing to see a counselor. Often, dating married men is a "safe" relationship. The expectations are low, demands are low and you already know that he is involved with someone else so the worries about him leaving you are slim. It also gives the woman some power since she is not as emotionally invested.
This all is an illusion, however. Your daughter has no idea who these men have been with or if she is the only affair they are having. It also leaves her with almost no chance of having a real relationship or a stable family if she should have children with any of these men. It also affects her spiritually, if she has any beliefs. She may already be aware of all of this but not be able to resolve how she is feeling. She needs to be motivated to change her behavior and hopefully she will be. If so, counseling would help her make better choices for herself.
I hope this has helped you,