Using Musescore 3.1.0, I am able to produce triplets from a dotted sixteenth note. First, you should have only two sixteenth notes in your measure, not three. Below is a diagram showing how it is done. Start with two sixteenth notes. Then go to tuplets, and then select triplets and you shoud see the sixteenth note split into three notes with a bracket underneath with a small 3 in the bracket. The first triplet will be the same tone that you clicked on while the other two are shown as rests. Then enter the notes that are desired for the two remaining triplets. You an also cange the first triplet as well, if desired. If this doesn't work I sugest that you reload the Musescore program.
Ctrl+3
on a quaver, then.
to get dotted semiquavers.