For this situation, you would use the AfterUpdate event of the combo and based on the value of its bound column, modify the recordsource of the subform. Your goal is a little bit different from what's usually asked for, which is to filter a subform based on a combo value. In your case, you want to "append" the choices so that it's A or A+B or A+B+C, so the solution is a bit more complicated than the other scenario. I'm assuming you never want A+C or B+C.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.