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.
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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) ->