i am assuming you are asking in relation to your nikon D50. the nikon BR-2a can be coupled with the BR-3 when using a 52mm threaded lense (e.g. AFD50f1.8) for your reverse mount setup.
[nikon cam body] - [BR-2a] - [50f1.8 (reverse)] - [BR-3] - [52mm filter]
hope this helps.