Is there a word for this three-pronged shape?

It's an adjective, but there's triradiate:

having three rays or raylike processes.

So far triradius has been sequestered in biology (and palm reading) as a term for the whorl on the human palm at the base of each finger.

I would probably use Y-shape as suggested before if clarity was most important. But triradius/triradii has a nerdiness/technical feel to it that could prove to be just the thing depending on context.

Tri-point is the name of the screw/screwdriver shaped like your first image.

From Wikipedia:

The tri-point, security screw head is similar to the Phillips screw head, but it has three points rather than four. These specialized screws are usually used on electronics equipment.

The second image is more of a star. It might be more precise to say three-pointed star in that case.

That's the term that Mercedes-Benz uses for their trademark:

How it all Began 1909: The three-pointed star on all routes.

Given that the trademark is over 100 years old, the term is very well known.

Both of these terms will be understood by most (if not all) English speakers. They can also refer to any orientation of the shape.