How do the missing cards split?
Before any combination makes sense, you need one idea: the cards you and dummy don't hold are divided between the two defenders, and some divisions are more likely than others.
Two rules of thumb cover almost everything:
An odd number of missing cards tends to split as evenly as possible. Five missing cards split 3‑2 two times out of three. An even number tends NOT to split perfectly evenly. Four missing cards split 3‑1 more often than 2‑2 — that surprises everyone at first.
Percentages are exact, before anything is known about the opponents' hands. Bidding or early play by the opponents can shift them.