A question about dialplan

hi all,
I have a question about dialplan, Are there any standard channel variables which keep both calling number of a bridged call?

For example, there are three extensions, 201,202 and 300. when 201 is having a conversation with 202, 300 wants to know who is talking with 201 now. in this case, which variable the 300 need to query to get the peers of the bridged call?
I know there is a variable called “${BRIDGEPEER}” which seems to be only queried by one part of the bridge call. but I need the third part who can query it.
I expect I describe the question clearly, and really appreciate your help!