Caramel, Cara and Crystal

What is the difference between these three malted grains?

Nothing. They are interchangeable terms.
Caramel, Cara and Crystal describe a collection of malted grains that have one extra step in the malting process, stewing or saccharification.

Saccharification involves heating the moistened grains before the drying process to about 158 F (70 C) for 1-2 hours with very little ventilation to reduce drying. This process lets starches in the malt become sugars, a process called Maillard reaction.

The reaction is responsible for a large selection of the malt’s aromas, including toffee, caramel, toast, nuts, raisins, and sherry to name a few.