An elaborated article about RAM:
Do you have a computer with four RAM slots? If so, you can use three of them. The fourth slot is for a graphics card, if you have one.
You can use the slots in the following order: primary memory, graphics memory, secondary memory.
There are some computer cases that have only three RAM slots. If this is the case, you can’t use the fourth slot for a graphics card.
In this case, you would have to buy an expansion card.
If you have a computer with four RAM slots, you can use them in the following order: primary memory, graphics memory, secondary memory, and expansion memory.