How to transform rhythm subdivision?

How to change an eighth + 16th +16th to triplet of eighth?
If I click the first to be another 16th, then there will be an automatic rest inserted (why?),
my intuitive solution has been turning the other two 16th into 8th, and then turn them into triplet, however MuseScore doesn't support this. What is the procedure in Musescore?
I see from a post one said only way is to turn the first note to quarter, but this would absolutely not work on a large scale when I'm changing multiple sections.