First, save a shape for each shape you want, the order has no importance.
In the Mixer, remove any keyframes that shape tracks might have.
Then create a custom paramater set. Add a slider. This slider will control the three shapes.
Now you have to link the parameters. Put the shape A's weight at 1 in the Mixer, and the other at zero. Put your custom parameter slider at 0. Right-click on the animation divot of the shape A, and choose Link With. Browse to the custom parameter set, select the custom parameter, and click anywhere else to validate the selection.
Repeat this for shapes B and C.
Now, put the custom slider to half it's lenght. In the Mixer, put the weights of shape A to 0, shape B to 1 and shape C remains at 0. On each divot of these tracks, right-click and choose Set Relative Value.
Finally, put the custom slider to 1. Leave the shape A slider to 0, put the shape B back to 0 and put the shape C to 1. Again, set relative values on each divots.
You might have to edit the interpolation in the Animation Editor for each track.
Hope this helps
Salutations - Cheers
Bernard Lebel
BTW: Don't want to sound haughty, but this is rather an animation question, no?