git merge a b c does an octopus merge — one merge commit with N parents. Used historically by maintainers to land batches of small unrelated topic branches.
Constraints: octopus refuses to merge branches that conflict. It's strictly for clean fast-forwards over a common base.
The default strategy is octopus automatically when ≥3 refs are passed. To force a regular pairwise merge, do them one at a time.
You'll see octopus commits in the Linux kernel and other large open-source projects. In day-to-day product engineering they're rare — but worth recognizing when you spot a commit with 4 parents in the log.