No need for fancy formulas or whatever
Enter the value 3 in any empty cell and copy it
Select the range(s) where you need the value added (entire range(s) adjacent or not)
Right click and click Paste Special
Select Add under the Operation header - Ok - Done

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Using O365 v2503 - Did you know you can thank someone who helped you? Click on the tiny scale in the right upper hand corner of your helper's post