The easiest way would be via tee http://man7.org/linux/man-pages/man1/tee.1.html and a named pipe. http://man7.org/linux/man-pages/man1/mkfifo.1.html
Create a fifo
$ mkfifo temporaryfile
Insert the tee into the middle of the pipe outputting to that file
$ command | tee temporaryfile | othercommand &
Feed the contents of the fifo into your third command
$ thirdcommand < temporaryfile