You know, I understand they have all worked together for a very long time, and they need to vent - but I think they should let me work out my own working relationships with others without trying to give me pre-judgements about those persons.
I don't think that managers should vent to other employees, especially 3 times a week. What are they venting about? Work quality? Personal problems with the employees? In the end, you don't want to engage with this back stabbing/talk, so refuse to at all costs - yet do speak up about the weather the tasty bean dip, etc.
I agree. If they need to vent, they need to vent to a psychiatrist or their spouse or someone unconnected with the company. What I'm hearing from the OP sounds like a "mean girls" group which is creating a toxic environment. If the team leader complains to underlings about someone who is not there, how are the underlings going to look at that someone in the future? Are they going to be a little less willing to cooperate when they know that the team leader disapproves of that someone?