Flipping a 3D Triangle on all 3 of it's sides.

There are two options. The first is to simply use the Pan Behind tool (y) to move the anchor point between rotations and then animate position to counteract the movement when the anchor point is moved.

For example: Create a Polygon, change the number of sides to 3, Adjust the radius to give you the size you want, convert the Polygon to a Bezier path, make sure snapping is turned on, use the pan behind tool to move the anchor point to the bottom right corner of the triangle, then set a keyframe for rotation. Here comes the repetitive part:

• Move down the timeline 20 frames and set the rotation to 120º
• Set an anchor point keyframe and a position keyframe
• Move down the timeline another 20 frames
• Grab the Pan Behind tool and slide the anchor point along the bottom of the triangle until it snaps to the next vertex
• Set keyframes for position and rotation
• Move down the timeline 1 frame and use the Pan Behind tool to move the anchor point to the next corner
• Set position and rotation keyframes
• Repeat as necessary

The timeline would look something like this:

Here is a project file: Dropbox - Triangle Wheel.aep

The other option is to use an expression to basically do the same thing. You don't need to convert the triangle to a bezier path, but you will need to access the outer radius to figure out how long each side is.

I don't have the expression handy and I don't have time to write it out and test it this morning but here's how to rotate square wheels. I  made this post back in the CS3 days... Square Wheels

If my day slows down I'll try and post the expressions that you would need to add to anchor point and Position. With the expression method, all you have to do is to animate rotation and the triangle will move along a straight path.

If that is not what you want to do then please give us more details.

Without screenshots it's hard to tell what you're working with. When you say a 3D triangle do you mean a flat triangle in 3D space, or is it more like a pyramid? I'm leaning toward the first because you said "both" faces, but it would help to be on the same page.

As for a dynamic anchor point, if you wanted the triangle to rotate from different places you have a couple options.

1) Change your Anchor Point right before each move by keyframing (with Hold keyframes just to be safe) both the Position and Anchor Point properties to move it to a new location.

2) Option one works but can get super annoying, so instead make multiple nulls. Place one null at each corner or point you want to flip from and instead of animating the parameters of your triangle you have multiple nulls with animation. This gives you the most flexibility and control. It also means that adjusting one null's parameters won't screw up the animation created by another one.