FindOpenBabel3.cmake is not fully functional
(confusion between OpenBabel3_INCLUDE_DIR / OPENBABEL3_INCLUDE_DIR)
The easiest way is to amend here.
--- a/cmake/FindOpenBabel3.cmake
+++ b/cmake/FindOpenBabel3.cmake
@@ -26,6 +26,6 @@
     set_target_properties(OpenBabel3 PROPERTIES
       IMPORTED_LOCATION "${OpenBabel3_LIBRARY}"
       IMPORTED_IMPLIB "${OpenBabel3_LIBRARY}"
-      INTERFACE_INCLUDE_DIRECTORIES "${OpenBabel3_INCLUDE_DIR}")
+      INTERFACE_INCLUDE_DIRECTORIES "${OpenBabel3_INCLUDE_DIR}/openbabel3")
   endif()
 endif()