In answer to Wolf, I'm not so worried about the C/C# in this particular tune, naturally. I shall come to another example shortly.
McDouglas, that is the fingering I tried originally, then wondered whether ST used 1 & 2 all the time. 2 & 3 seem more natural in that position, but feel weaker, although perhaps that is simply down to a need to practice and build up muscle strength.
I thought that I would share an excerpt from a modified "hornpipe" in jig form. It isn't in its original key and was modified to be played on a D/G melodeon. 212 in the left, then 121, 212, 121 in the right seem to work here since we have C not C#. The tune is in the key of G. What follows is D in the left. So far with searching I haven't found a clear example with C# and I think it's more likely to occur fleetingly for two notes rather than three in tune composed in D or partly in D.