I'm trying to set up conditional formatting in a spreadsheet where there are 3 possible correct letters that can be entered into a range of cells and if one of these letters is not entered then I would like the cell to be highlighted.

To be more precise, it's an accommodation spreadsheet and in each cell if someone needs a room for that particular night (each night is represented by a separate column) then a letter is used to indicate what type of room they require i.e. "k" is a king room, "s" is a Suite and "t" is a twin. I don't want to force people to have to select from a list which would be the obvious solution to this as it is simply too cumbersome for the amount of data entry we do.

However i want to ensure that if someone accidentally enters anything other than t,k or s then the cell is highlighted. We work in large spreadsheets covering months of dates so to manually have to find any errors is hugely time consuming.

I've tried setting up OR formulas and NOT formulas but can't seem to find a solution.