Conditional Formatting Based On Multiple Text Criteria?

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.