Ken - the 1+ is on the middle string, not the melody string. Tuning to DAA and adding 3 will just move it to the 1+ fret on the melody string(not the 4th fret). You will also need the 8+ to cover the 6's.
This is something that crops up in a fairly large number of tunes where the root note falls on the open string, so maybe it's worth spending a bit of time looking at ways of working around it if you're playing Drone style.
When the root note is on the open string(mixalydian) there is no leeway to go any lower on the melody string. In the case of SMTTF the tune calls for the note below the root, so you have to find some other way of getting this.
With no extra frets.
1) 'Fudge' it or leave it out. This works well for some tunes, but unfortunately, the note we're looking for here is the most important for this mode( a minor 7th), and it's not so easy to leave it out without changing the character of the song.
2) Move to the higher register(octave) i.e. move the root note to the 7th fret by adding 7 to the TAB (1+ on the middle becomes 6 on the melody).
Give this a try first. It may not sound particularly good, many dulcimers tend to lose something up the dusty end, but it will give you a good view of what the problem is when you go back down to the lower register with the root on the open string.
3) Borrow the note you want from one of the other strings.
If you're in DAD (key of D) then the note you want is a 'C' which can be found at the 6th fret on the bass. This is a bit of a stretch and probably not a very good solution.
Or you can take Cynthia's approach and move the melody on to the middle string (key of A) then the note you want is a 'G' which can be found at the 3rd fret on the bass.
With a 1+ fret.
4) Tune to DAA and add 3 to the TAB. (1+ on the middle becomes a 1+ on the melody).
5) Play as TABed in DAD, borrowing the note you want from the 1+ on the middle.
With a 6+ fret.
6) I've left this to last as it is often the most practical solution.
Tune to DAG(Dorian) and add 4 to the TAB. (The 1+ becomes a 3, and the 3 becomes a 6+)
7) Make any extra notes you need using an angled noter.