Is there a word for this three-pronged shape?

I suggest the term forked cross might be a good one to use. It does seem to be associated with a slightly elongated version of your image, but it seems in common use and might help you out:

http://images.google.com/?q=%22forked%20cross%22

"A forked cross, is a Gothic cross in the form of the letter Y that is also known as a crucifixus dolorosus, furca, ypsilon cross, Y-cross, robber's cross or thief's cross." [Wikipedia][1]

I believe the geometric name for this is an Inverse Hexagon.

Try caltrop, it's the first thing that came to my head. A google image search will show you they generally look like this from the side. In fact, nearly your exact representation is used in a wikipedia article section about their use as a symbol.

Edit:

Ah, just noticed you were saying you didn't want something like the second picture, but the first, which my answer doesn't really help with.

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.

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

There are a lot of hits on Google for 'inverted Y-shape'. If you don't care about the orientation, you may go for the simpler (more generic) 'Y-shape'.