An (*almost) individual 3 X 1 is the easiest to make, stealing the port runners from an existing aluminum manifold and adding a flange to each cut end. Many possible choices but all are simultaneous operation so size & jetting are critical, each cylinder pair needs more CFM than you might suspect.
* frequently, a small balance tube is added
A top with multiple patterns means a common plenum, so the carb CFM is cumulative.
