I'm building a simple booking system using GoogleSheets. Each entry (Row) is a "booking request". The intervals being entered on the sheet (via a form) are fixed to 1, 2 or 3 hours (which makes this a "simple booking system"). I'm validating these "booking requests" based on "overlaps" in Start & End Time in a couple separate columns L, M, N.

Then you can test whether the result is non-zero and display a message:

=ArrayFormula(if(A2:A="",,if(countifs(C2:C,A2:A 1/24,row(C2:C),"<>"&row(A2:A)),"overlaps 1 hour",)))

and similarly for 2 and 3 hours.

BTW I don't think row three does overlap at one hour.