The definition of a stay from the honors T & C's: "A "stay" is defined as the total number of consecutive nights spent at the same hotel, whether or not a guest checks out and checks back in again." To earn points: "To earn Hilton Honors Points or credit toward tier status for hotel stays, a Member must be a registered, paying and staying guest at a participating hotel within the Hilton Portfolio and meet all of the conditions described herein."

If you meet the conditions to renew your Diamond membership by the end of this year, your Diamond membership will last until the end of March 2020. You certainly can stay in hotel A night 1, B night 2 and back to A for night 3 to get credit for 3 stays. You can't check in to 2 different hotels on the same night as only one of those would register for a stay credit. You can check in online via the app and chose your room the day before check in, but that often limits your chance of getting an upgrade. I prefer to see if an upgrade has been offered on the day of check in and if not wait until we arrive at the hotel and sometimes do a bit of grovelling! Even at low rates in the £50 to £60, we have had some nice upgrades in the UK. Good luck and hopefully you have fun!