There is the Harmony feature in BiaB, e.g., Harmony->Melody Harmony (select) which offers a number of ways to harmonise. A number mention 3rds. Does that not do what you want?


Jazz relative beginner, starting at a much older age than was helpful.
AVL:MXE Linux; Windows 11
BIAB2025 Audiophile, a bunch of other software.
Kawai MP6, Ui24R, Focusrite Saffire Pro40 and Scarletts
.