Mary Knew Phyllis BUT ...

Rhoda was an unknown to Mary as the 1st episode shows, battling for the apartment with Mary

than BANG

Rhoda is like her best friend

I'm not sure just what transpired

The show never really showed Mary and Rhoda "becoming" friends (?)

Was it a season 1 writing over site ? or did I miss something ?

Phyllis was older than Mary and Rhoda. Phyllis was a neighbor and the Manager of the apartment building. Of course, Phyllis was another source for story ideas. Mary did know Phyllis from before, but I am not sure how they knew each other from before.

Towards the end of the first show Mary and Rhoda both agreed that they would have a hard time disliking each other. As time went on they became stronger friends, rather than just neighbors that you may be friendly with.