Is there a way to specify a tuplet of three 8ths in a bar of 5/16?
You’ll need two levels of tuplets, with the lower level hidden. So, first make (for example) a 6:5 sixteenths tuplet, then a 3:3 eighths on top, with the 6:5 hidden. Or 4:5x (hidden) and then 3:2e. All functionally equivalent and will play back correctly.
Try a 6:5 16th note tuplet, then input three 8th notes?
Edit - hrnbouma’s idea is better.
There is an easier way!
Make a 6:5x tuplet and Properties > Tuplets > Use Contracting Ratio.
I note that I do have to force duration for the middle of 3 eighths.