What could it be ?

I just recieved an older DS-90 Refractor. The lens cell seemed to be a little large for the tube. There should be two lens but there are three. It gives very good "no color and darn near perfect" views as is but I cannot collimate it. And what is the third lens for ? When I attempt to collimate it with a cheshire I see three donuts all apart from one another. With some very careful work I have them very close to being in the center. Because the cell is a bit to big, its sloppy. Also that 100% plastic focuser has to go, anyone know what will fit it or fix it ?