With adding a center channel, it is important that its sound characteristics match the front right and left. This is called using a "voice matched" center. Pretty much every brand of speaker these days sells various models of speakers that are "voice matched." Ideally, you would use an identical speaker for the center channel. The reason is, when a sound pans across the front, you want its tonal character to stay the same and not change as it moves from one speaker to another.
Ideally, the center speaker will be at the same height as the right and left front, and ideally it will be in the center of the screen itself (which is only possible with a front projector with an acoustically transparent screen, with the speaker behind it, which is what is done in most movie theaters), but immediately above or below the screen can be fine (in my main home theater, mine is immediately below the screen). Whether above or below is better, depends on the height of the screen and on your seating; generally speaking, you want the tweeter of the speaker as close to ear height as possible, and you want the tweeter as close to the TV as possible. So if the top of the screen is closer to ear level, then putting it above the TV would be better. If the bottom of the screen is closer to ear level, then putting it at the bottom will be better, unless there is something that would block it for your seating position (as would often be the case if you had two rows of seating; the first row of seats may block the sound if it is below the screen). Everyone should be able to see all of the front three speakers from their seating position.
Now, how important a center channel is depends on several things. One of which is how far apart you place the right and left speakers. The further apart they are, the better it is to have a center channel speaker.
The second thing that affects how important a center channel is depends on seating. If you only sit in the middle, half way in between the speakers and looking directly at the center of the screen, then it matters less than if you are watching a bit off to the side.
If you cannot put the center speaker exactly centered with the screen, either just above or just below the screen, I would not bother with a center channel speaker at all. And I would not bother with one if it was not at least "voice matched" to the front right and left.
If, when watching with your two speakers, it ever seems like the voices are not coming from where they look like they are coming, then it is more likely that you need a center speaker, though a misplaced one could make it worse.
I really do not feel the need for a center speaker with my bedroom system, but I have the speakers as close to the TV as possible, and my expectations are lower for that system than for my home theater. It gives me clear dialog, with a much better sound quality than the speakers in the TV gave me (which I could not stand at all, which is why I have the speakers there).
The only way to know how much you will like it will be to try it. But I would want to know more about what you have and how you use it before I would advise you to get one.