Adapting the conditional formatting formula for cross-references

I currently have a sheet configuration for the job that compares 3 columns of data with conditional formatting. The goal is to identify if the same string can be found 1 or 2 times on the 3 columns. I used the following conditional formatting formulas = countif (A: C, A1) = 1 and I have 2 other formulas with = 2 and = 3 at the end. It works perfectly for my needs, except that I would like to be able to sort / filter each column separately, which Google Sheets does not seem able to do. So my thought was to separate each column into a separate sheet. The only problem is that I can not find the appropriate way to transform the conditional format into cross sheets and give me the results needed to identify the same information one to two or three times spread over 3 sheets. I've tried something like = countif (Sheet1 !: A, A1) = 1 but that does not seem to be a valid formula and I should check the 3 sheets, not one. What I basically want to do is replace the A: C part of the original formula by Sheet1: Sheet3 but I do not know how or if it is possible. It may not even be a possibility, but I thought it was the place to ask.