Ok so in this experiment what we want to find is the catalyst for the reaction. Our original reaction is the following. A + B --> C, therefore to find which of the unknown substances (X, Y, and Z) is the catalyst reaction we simply have to do the following:
A + B + X --> C
A + B + Y --> C
A + B + Z --> C
The reaction that produces C much faster than our original reaction A + B --> C will be our catalyst. The reason being that one of the substances will increase the rate of the chemical reaction.
I hope it helps, Regards.