A rough sketch of a possible speaker position according to your suggestion. Center is 90 degree coverage and side fills are at 60 degree. How far down or up would you place the side speakers? it seems to that not all the seats will get a good coverage upfront .
That is not what I meant.
The side speakers should be to the sides of the main and pointed down and towards the walls.
This helps to maintain the "expanding wavefront" of the center speaker.
When you have patterns that start to go in different directions (as shown) you start having all sorts of interference.
Think of it as pebbles in a pond, you want the main ripple to be continued-not interfered with.