Twins are not slow learners. It has been observed that they become a self-sufficient pair, communicating between themselves very effectively, through gesture and speech. When they attend a play school or become part of a play group, they will realise that others are not so quick to understand them and will begin talking more. Twins have been known to prefer each other's company and to do the same things, with no previous verbal statement.
Since your son also spoke less than his peers and has now caught up with them, it is to be expected that the little girls will also catch up. Get them children's music that they like and let them dance and move to the music. They will pick up the words of the songs. Let them look at picture books and listen to children's stories that you read or tell. They do not need a speech therapist, in my view.