Basically, I want to merge single cells in a LyX table without using ERT (hopefully) to get something similar to:
To clarify, cells of Headline 1 and 2 are merged vertically and Headline 3 horizontally.
Thank you.
I found the answer myself.
In Windows, select cells in question > Right click > "More..." > "Multicolumn" or "Multirow".
Lyx 2.3: Select cells -> Right click
-> Columns/Rows
(at the bottom) -> Multicolumn
/Multirow
.