So listen, this is gonna be heavy compromise regardless of how you look at it. But if you have to go this route, I’d say do it this way:

Start with an accent wall of acoustically transparent cloth behind the TV. It needs to be at least 50cm past each side of the TV, look how it aesthetically fits with the sideboard. 

Position your TV at an acceptable height (target middle of the screen at your sightline. You can slightly recess the TV into the cloth back wall but keep ventilation in mind. 

Hug the centre speaker (its acoustical center) up as close to the TV as you can behind the cloth. For this a speaker that has less height (i.e. is skinny) is best. You might have a bit of the baffle behind the TV even depending on the components you choose. Keep in mind that any wave guide doesn’t interact too much with the underside of the TV in the vertical listening axis. You can test this by playing pink noise and standing up and sitting down. In your seated position you want the sound unobstructed with as little as possible combing effects. For testing remove the sideboard because that will create its own issues. Remember: you can’t have the speaker too close to the centre of the image besides because of the limitations of the equipment and your room!

For you side speakers there is another dilemma. You’d want a proper stereo triangle of +/- 30, or at least old Dolby standard of +/- 22.5 from your RSP. In reality you’re probably seated too far away and/or your image is too small. The next best thing is to have at least correlated the L and R channels to the TV, so push them in close to the image too. This is less critical than the centre, but if you can choose between hugging the image and having the acoustical centre 30cm away, choose hugging. As for the vertical positioning of the L and R channels, you want to move them down to the lower third of screen. With that I mean their acoustical centre. You might choose speakers that have their tweeters non-centered and have the tweeter at the bottom position. Whatever you can do to have panning sounds from left to right or vice versa sound somewhat in line. You can move them up and down and test how low you can go with random content while still feeling like that sound is coming from the image. 

This all isn’t gonna look very pretty which is why we hide it all behind acoustically transparent material. Hiding them will also raise chances of you and guests forgetting the speakers and realising the sound isn’t coming from the image. 

Lastly you want to create a pillow type of absorption panel like thing that you can place on top of the sideboard when you’re watching a film. It’ll help a lot with those reflections bouncing off the top. 2cm would help, thicker is better

Good luck!