Hi @DoEDorkily123,
Welcome in the Community !
When setting up a Mixture design through the Custom design platform, you can specify the total amount for your 3 mixture factors. Click on the red triangle next to "Custom Design", then "Advanced Options" and then "Mixture Sum". Here you'll be able to specify that the total amount of your three mixture factors should be 0,15.
Note that with this constraint, each mixture factor range goes from 0 to 0,15 (15%) by default (unless you specify other constraints).
Hope this response answers your question,
Victor GUILLER
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)