ZLC Center Controller reported issues with A320 aircraft that are receiving erroneous TCAS RA's when there is no other traffic around the aircraft.

Narrative

I am at ZLC and have received 3 reports in the past 3 days for A320 responding to TCAS RA's when there was no traffic in close proximity. Here are the details from those reports:Flight X was on present heading climbing to FL350 for traffic Flight Y descending to FL300. Flight X reported a TCAS RA that they were descending for; but didn't have an altitude on the traffic. Traffic was issued Flight X on Flight Y and informed them that they were going to pass eight miles apart. Flight Y asked if there was any other traffic for them and we reported that there were no other traffic that was a factor. Closest proximity to Flight Y was 9.33 miles and 1;800 feet.Day 2 XA40Z Flight Z (A320)Flight Z checked on with ZLC Sector 40 and reported that they were responding to a TCAS RA. Controller asked if they knew what it was for; and Flight Z responded that it was for traffic below them and they didn't know if it was a false one or not; but they were starting their descent back to FL350. R40 responded that they were showing nothing on their scope.Day 0 XX06Z Flight A (A320)At XW:06:45 Flight A advised R44 that they received a TCAS-RA while climbing from FL290 to FL330. No traffic in the vicinity. Closest traffic was at FL240 at 9 miles and FL380 at 12 miles. Flight A did state that they didn't get a traffic alert and it went straight to a TCAS-RA. R44 advised Flight A of the known traffic. Flight A advised that they were able to return on course and altitude with no interruptions.Recommendation - Check and see if there was an update with A320's in the past few days.