An illustration and some excerpts from an aerodynamic research paper on the Y-20 tanker, confirming that it will have three fueling points: one under each wing, and the third being underbody on the centerline and toward the rear.
Refueling behind another aircraft is already a shaky business. They might have found issues with the vortex creating jet wash for the refueling aircraft as it comes in behind the wings.
With Y-20A 11155 - aka no. 15 - assigned to the 12th Air Regiment, 4th Transport Division at Chengdu/Qionglai the so far highest individual number was spotted.