According to the CMake documentation:
target_link_libraries(<target> [lib1 [lib2 [...]]] [[debug|optimized|general] <lib>] ...)
A “debug”, “optimized”, or “general”
keyword indicates that the library
immediately following it is to be used
only for the corresponding build
configuration.
So you should be able to do this:
add_executable( MyEXE ${SOURCES})
target_link_libraries( MyEXE debug 3PDebugLib)
target_link_libraries( MyEXE optimized 3PReleaseLib)