We need to identify which connections are which solder points on the plugs. Use your multimeter on the resistance setting again, and place one probe on the tip and the other on a solder point. Keep trying until you find the corresponding one, which will register around 0 ohms.
Mark these down on a piece of paper so you remember which is which. As follows:
The TIP is the LEFT channel.
The MIDDLE is the RIGHT channel.
The BASE is the GROUND.
The jack illustration below is from http://www.ehow.com/how_114206_replace-headphone-plug.html
Take your headphone plugs and solder 3 inch lengths of wire to each connection in them. Solder a black wire to the ground connection and two red ones to the left and right connections.
Solder these wires in place on the headphone cable. Solder all three plug ground wires to the single ground wire from the headphones. The diagram below shows the connections.
When I was complete I encased mine in a small plastic container. A large diameter heat-shrink tube would work well also. Plug each one into a MP3 player, one by one, to make sure they work. You should hear some amount of sound in each ear as you test them. If you're worried about damaging your MP3 player, put your multimeter on each jack connection and the speaker itself, and recheck that each connection is only connected once, and that there are no shorts to the ground connections.