I put the camera with the 18-300mm in the center. And I put the other 2 lens in the 2 outer pockets. Since one is a 35 mm lens and the other a 18-105 mm lens I can put a horizontal spacer in one of the side pockets to stack the two lens. Then I can put the flash in the other side pocket. I place accessories (chargers, etc) in the mesh pockets or the front pouch. Net, I have not seen any issue with getting my stuff in the Retrospective 7.
I do have a 70-300mm which I probably will sell now that I have the 18-300mm and a 80-400mm which I do not use often enough to carry in this bag. The 80-400mm has become my preferred lens for backyard bird photography so it does not travel much with me (plus it is heavy).
I also have a Tamron 90mm macro and a Tokina 12-24mm wide angle. They are specialty lenses and only used for occasions I am doing that type of photography. So I pull out the 18-300mm or 18-105mm and replace it with either of these lenses.
Net, the Retrospective 7 is just right. I thought about the 10, but decided I did not want to carry that big of a shoulder bag. Plus, I got a great price on the 7 at a local photography shop since they had too many.