Delta is not just T3. It also uses T2. (If international, TBIT.)

If taking a shuttle bus, any bus using the Pink stop will work--taking you from T2/T3 to T6/T7. People will get on and off at each terminal. The Airline Connector may be the least crowded.

If walking all the way along all terminals, lower level is probably better pushing luggage carts. Upper level often has crowds blocking the narrower sidewalk due to the various bus stops.