In the second custom field, you should be able to declare the dependency.
At the bottom, after the section where you enter the options for selection (1, 2, 3, 4, 5 & 6) you should be able to check a box that says 'dependent on', which will then allow you to select the 1 field you created (A, B & C). You will then be offered a grid to select in the 2nd field which options are available for each of the options of the 1st, so 1, 2 or 3 if A was selected, exactly as you requested.