TM-3 recalibration procedure

It depends on which version of TM3 you have. The older ones have only 2 trimpots inside―SPAN and OFFSET. You have to sync the two oscillators with the sync switch. They should track fairly closely. (If not, the tubes might need to be replaced.)

Run the TM3 output to a chromatic tuner (or amp if you have perfect pitch). Set the TM3 front panel tuning knobs to the exact middle of rotation.

1. Feed the TM3 a control voltage to correspond to the low C, from a Hz/v MIDI/CV and keyboard (or whatever you are using to control the TM3).

2. Adjust the offset trimpot (toward back of the PC board) to get a low C output pitch. 

3. Then feed the TM3 a CV for the C one octave up. 

4. Adjust the SPAN trimpot (closer to the outer edge of the PC board) to obtain the C one octave up. (Yes, the two oscillators might be out of sync. Just get it as closely as you can.)

Repeat steps 1-4 above until the lower and upper Cs are in tune.

If you have the newer version with 4 trimpots, the procedure is the same, except you have to disable one oscillator (by turning down its offset trimpot until it is silenced, then adjusting the other oscillator as above). Then, when one oscillator is in tune, you repeat with the other one. It's easier to do the second one, because the first oscillator can be used as a guide. Just tune the second one to match the first one.
Usually the trimpots are UNDER the board. The two toward the center of the board are the SPAN trimpots, the other two are OFFSET. One oscillator is controlled by the two trimpots toward the outer edge of the PC board.