It sounds like you are using Speedy Entry Tool to enter the triplets via the triplet shortcut (Windows:) Ctrl-3 / (Macintosh:) Option-3. Finale will base the triplet value on the first value you enter after the triplet shortcut. Since the first value you enter is a quarter, Finale makes the triplet a "3 quarters against 2 quarters" triplet, not a "3 eighths against 2 eighths" triplet.Workaround 1 (works with all mixed-value tuplets, 5 steps):
(Windows:)
Ctrl-3 (I want a triplet...)
4 (namely a "3 eighths against 2 eighths" triplet)
LeftArrow (however, the first value in the triplet should not be an 8th...)
5 (but a quarter)
4 (finished)
Workaround 2 (only for "3 eighths against 2 eighths" triplet, 4 steps):
(Macintosh:)
Option-1 (I want a tuplet...)
Enter (and Finale's default "3 eighths against 2 eighths" triplet suits me fine)
5 (first a quarter...)
4 (then an 8th)
Peter