Some of the DC amps have tricky footswitch circuits.
If you have a really early one, you might have to call Mesa.
You could try to get an original on Ebay, or get one from Mesa, but that is a bit expensive.
Here is what I did:
www.tubesandmore.com
P-H470 is the footswitch with 1 button
P-H471 is the footswitch with 2 buttons
P-H471-LED is a 2 button with LED
I had a DC-5 with LED's on the faceplate of the amp and the P-H471-LED worked just fine. The LED for the channel select worked great, but the EQ LED did not work.
The way to tell if you need a 1 button or a 2 button footswitch is to look at the back of the amp where the Reverb footswitch jack is.
If it only says Reverb, then you need the 2 button for the front jack. This would control the channel select & the EQ.
If it says Reverb & EQ, then you need a 1 button footswitch for the front jack, and a 2 button for the rear if you want to be able to switch the EQ in and out.
Now, here is the tricky part....
If you happen to have the Revision A version of the DC-3, you may need to add a 330k ohm resistor in series with the LED inside the footswitch. I had an older DC-2 that I had to build a footswitch for, and it ended up needing the resistors.
Hope this helps.