Currently it is not possible to change your tuning beyond the predefined ones.

Since you're using Pianoteq's resonance as an audio effect, a workaround for making the tuning contain only Es, As and Ds is to configure an unequal temperament in which all notes are almost Es, As or Ds.

Another option would be to enable Reserved MIDI channels for individual guitar strings, set the velocity curve to zero, and send infinitely holding MIDI notes on each channel for the resonances you want.

Let me know if that helps.

I am currently using Pianoteq as an effect for its sympathetic resonance in one of my projects. I've routed a sample library of a guitar-like instrument called the Balalaika which only has three strings, typically tuned to E4, A4, and D5, and I'm interested in either changing the tuning of Pianoteq's classical guitar to just Es, As, and Ds, or better yet, to both reduce the number of strings to 3, while also hopefully changing the tuning to be authentic to a real Balalaika. This is not a necessity, as just adding even the standard classical guitar resonance to the sample pack has greatly elevated the realism of the patch, however I just happened to wonder if any of these fringe things are possible.

I saw that you can choose from several tunings in the Guitar, however none of them fit my desired profile, unfortunately.

Thanks!