Held note or tenuto fingering indicates that a finger should be kept pressed on the fret also after the actual duration of the note. This is usually suggested to facilitate the move of the fingers to the next position. It is represented by a digit (the finger) followed by a t (meaning tenuto).
If you use \finger "3t", you will get a poor result, because \finger uses the Emmentaler notation font, which has numbers but not letters. As a result, the number will use a serif font, while the letter t will use a san serif font. The letter t will also be positioned incorrectly.
This snippet contains a function which creates the desired fingering object without above problems.