rerouting teleporters

Ok I have three teleporters at each spawn point...teleporter1 goes to
dest1, teleporter2 to dest2, teleporter3 goes to dest3. I want to make
it that when one person goes through a teleporter(for this example I'll
use teleporter3) all the other teleporters rerout their destination to
dest3...how can I do this? I looked at the triggers tab but there was
only one option in the middle missing half it's text.

Check out the
trigger_changetarget
entity. After the player goes into teleporter3 (your example) call the
trigger_changetarget. That should then change the target of all three
trigger_teleports to dest3.

For your example one way would be to create three
trigger_changetargets. Put a trigger_multiple where your teleporter
should be. That should target a multimanager. The multimanager should
call the trigger_changetargets, and a separately placed
trigger_teleport entity.