Removed Everything, rebooted, found the issue, quite simple to replicate actually:
Remove everything from the 3rd column... Voila 2 columns with default engine.
Only way to get the 3rd back is to remove the cookies.
Extreme zoom out used to show no 3rd column is added if column is removed.
It is so easy for me to encounter this bug as my laptop is actually 16:10 so 1920x1200 so at 100% zoom I see 2 columns by default, to be able to see 3 I need to go to at least 80% Zoom in my browsers.
I mentioned this as I noticed your screen is a standard 16:9 FHD, however it works as described on my other laptop with 16:9 it's just not as easy, as it simply "works" in 3 column mode by default.
P.S.: For the above reasons, I actually like the 2 column mode, but others might want a way to get back... It's a simple annoyance, to be honest.