If you have 3 comms boxes you can tune in to 3 separate frequencies. In each case the next anticipated frequency will usually be pre-selected on the box so that as you pass from one to the other you can just swap the active frequency. For example the active ATC comms may be set in box 1, with the next frequency waiting, box 2 may be being used to listen to the ATIS and box 3 to talk/listen to company ops. The pilots can choose which box(es) they listen to by selecting them on the Audio select panel. Although you can listen to several at a time (with practice) you can only choose one box to transmit on.
If there are two VHF nav boxes you can manually tune them to two separate VOR/DMEs or ILSs and both will be displayed on the ND. More commonly en-route the tuning control will be given to the Flight Management System(s) so that they can autotune the most relevant nav aids. the ND shows which navaids are 'active', or autotuned.