Tuplet problem - three quarters in two (only five eighths present)

I've got to copy a recent work into Finale. The work is quite heavy with quarter triplet sets in which one of the quarters is split into a grouping of two eighths:

.-- 3 --.
Q EE Q

When I try this, the set only gives room for five eighth notes (instead of the actual six present).

I have not worked with the program much outside of a basic orchestration course, and am not too familiar with how it functions.

How can get the program to do what I want?