I spotted these three planes flying east then turning south. They were over the Hopi reservation and heading toward Winslow, AZ at 10:42 AM local time (MST), today February 5 2021. They were at 15k to 20k feet or so. Can anyone explain?
The primary reasons large airliner-sized aircraft would stay in formation is for a photo flight, or for reasons related to military operations.
Very few civilian airliner-sized aircraft are grey.
The two grey aircraft are surely military tankers, both of the same type. The white aircraft is in position to receive fuel from the front aircraft. Based on color and large size (relative to the grey aircraft), the white aircraft is a 747-based military aircraft, such as a VC-125 (the type used as Air Force One) or an E-4. With this in mind, the size of the tankers suggests that they are KC-135's. A close look at the grey aircraft shows that they appear to have engines located well outboard on the wings, i.e. have four engines on the wings rather than two, further supporting this identification.
It is very common for military tankers to fly in a loose "trail" formation like this. I've seen it dozens of times. Whenever I see airliner-appearing aircraft flying in this type of formation I always reach for my binoculars and they usually turn out to be KC-135's, KC-46's, or KC-10's.
As to the "why" part of your question, a possible answer might be that two tankers operating together gives both crews a chance to practice refueling another aircraft of some particular type-- in thus case the E-4 or VC-125-- without having to put two different receiver aircraft in the air. There are many other reasons why the military often finds it advantageous to move several aircraft through the airspace together as a single formation rather than as completely independent flights.
FlightRadar24 is a good resource for answering a question like this-- it allows one to zoom in on a given area and play back tracks from a given time in the past. Much military traffic is not captured, but one could at least verify the presence or absence of civilian traffic at the time and place of interest. ASE user DeepSpace has now provided this link, showing that in this case the ADSB transponder of a KC-135R was in fact captured at the time and place of interest. The flight started and ended in Phoenix, and included two passes over the Hopi reservation. The reported altitude over the Hopi reservation at the time of interest was 22,075'.
The first two aircraft seem really close in lateral distance, but at much different altitudes. If they are separated by 4000-6000 feet of altitude (which could be the case), they are about a mile apart. That is a safe and respectable distance.
The third aircraft is about 30° (rough estimate) of displacement from the first two. Depending on the distance between the aircraft and the observer, this aircraft could b 1.25 miles from the first two. This is based on 2.5 miles of difference between the observer’s elevation and the aircraft’s altitude.