Look at your Motherboard's Support RAM MEMORY QVL LIST for your RAM or Processor.
If your RAM is listed it will show you how compatible it is with your Motherboard and processor.
Found this Tech site that answers your question about using 3 sticks of RAM in a motherboard: https://maximum-tech.net/can-you-use-3-sticks-of-ram/
You will observe fast browsing and multitasking as well, but it will be only better than your previous 2 stick RAM. This answers the question of using 2 sticks of RAM or 3. To understand it deeply, a motherboard usually comes with two channels or dual channels in which memory slots are present.
The RAM sticks are always correspondingly inserted in these slots. It is also known as RAM working as a dual channel. Facts say that adding 2 sticks increases the bandwidth to less than 50% on Intel and even less than that in AMD PCs.
You can now imagine the performance increase if you add a third stick in a motherboard with 4 slots. As this third stick works as a single-channel then of course it provides less efficiency than the previous 2 sticks of RAM and thus gives a slight performance increase to your PC.
This answers the questions of using 3 RAM slots out of 4. This same thing happens whenever you install multiple-sized sticks.
Basically two Sticks of RAM will work as DUAL CHANNEL MEMORY while the 3rd stick will only work as a SINGLE CHANNEL MEMORY thus you will get a slight increase in performance with 3 sticks installed over having 2 sticks installed.
4 Sticks would be ideal since each 2 sticks of RAM will be working as Dual Channel Memory instead of Single Channel Memory.
