Create a Summary Report from a Multi-select Dropdown Field

You can do this with three fields.
The formula for [A count] would be

If(Contains(ToText([my multi select field],"A"),1,0)

But be sure that your choices are unique like ABC and not like

Mice
Elaphants
Ants


Because the phrase ants is also in elephants.

Then do similar fields for the [B count] and [C Count]