Is the 3 in. Galvanized Steel Adjustable Pipe Flashing with Base and Rubber Collar suitable for a flat roof?

It can, but it would need to be sealed well around the base to prevent water from entering in the event of a puddle.

It can, but it would need to be sealed well around the base to prevent water from entering in the event of a puddle. It is difficult to control the direction of water flow on flat roofs compared to pitched roofs.