Show the DL agent at MEM your CX itinerary for HKG and they should check your bags thru to HKG for you.
In the rare case that they will not (I'm pretty certain they will), three and a half hours would be plenty of time to claim the bags and walk over to TBIT and check them in with CX. From the Delta terminal, just walk toward the AA terminal (T-4) and then another hundred yards or so beyond the AA terminal is the TBIT.