You will have seen that the coax to pines 3 and 4 has a crossover between left and right connector. For fault finding in the 3T77A I would do as in the time base connector, that is with braid to pin 4. Don't connect the braid to the ground wire! The other coax at pins 18/19 is straightforward.
BUT as far as I can see the 3S2 makes no use of pins 3/4, so cannot trigger the 7T11A internally. In the 3S2 diagrams you can see that the trigger signal merely goes to the Trigger Out front panel connector.
In my extender I used ordinary wire, not twisted or whatever. Remember that you normally won't use the extender during (HF) calibration. IIRC all adjustments can be reached from the left and right side of the oscilloscope.