You are using the “full Finale” (not one of its smaller siblings) - right?

By the Way:

What version of Finale?

Yes, there is a way.

I suppose that you by “mountain dulcimer” mean a dulcimer with diatonic fretting and 3 strings - right?

Window menu > ScoreManager > Instrument List

Add a Tablature staff for the mountain dulcimer TAB.

You may have to customize the TAB staff after you have added it:

Utilities menu > Change Instrument…

If you (in Change Instrument) choose the sub-menu Tablature, you will get a long menu with TAB instruments, including 3 choices for Dulcimer.

I am not sure how detailed instructions you need (are you an experienced Finale user?)

If you have more questions, let me know.