Splitting, merging, splitting, merging... this is a common way to get into trouble with INDD tables.
Remember that each time you split a cell, indesign creates a new "ghost column", and because of minimum inset, you'll never be able to align it with the regular column. Besides, they are not related.
Solution is: Do not use splitting. Use Cancel merging instead.
See example below:
Many thanks to both of you - both replies were extremely helpful & I've now managed to fix it :-)
1 person found this helpful
perhaps you see this and wondered how to solve it:
unmerge > merge
will not maintain formatting of cells ( insets, fill color, stroke weights etc.pp. ).
I suggest some additional steps:
Select the three cells before unmerging.
 Copy the selection to the clipboard
 Unselect everything ( KBSC on the Mac: cmd+shift+A )
 Paste to the page > a new text frame with a new table and the 3 cells will appear
That step is required to collapse any number of columns in the copied cells of the clipboard to exactly 3.
 Select and copy the pasted 3 cells to the clipboard,
 Remove the pasted text frame
Select the merged cells:
Done. The 3 cells's formatting stayed intact.