Can't arrange 3 displays with 2 on top and one below

I have a 2013 MacBook pro with Catalina (10.15.7) and have added 2 external displays, one on hdmi, one with a thunderbolt VGA adapter. All 3 (the builtin LCD and 2 external) are used and not mirrored. When I try to arrange the two external displays side by side above the LCD in the displays settings "arrangement" tab, it refuses to move the third one (whichever I arrange last).