You can search for a string in any text-window of KDiff3. The Edit → Find... menu item (Ctrl+F) opens a dialog that lets you specify the string to search for. You can also select the windows which should be searched. Searching will always start at the top. Use the Edit → Find Next... menu item (F3) to proceed to the next occurrence. If you select to search several windows then the first window will be searched from top to bottom before the search starts in the next window at the top again, etc.