Air carrier F/O reported on three separate occasions the aircraft deviated from the DEN RNAV (RNP) Z 17R approach track.

Narrative

Flight A; Flight B; Flight C.I was the Pilot Flying on all 3 of these legs and all were the RNAV (RNP) Z 17R in DEN. In all three approaches just inside of JOULE FAP on autopilot; the aircraft began a turn from the runway centerline. The first two times it was to the right and the third time was to the left. All three approaches were conducted in VMC; one night and two day. First two times I disengaged the autopilot/auto-throttle and hand flew to landing. Having seen this twice and with the Captain's concurrence; I allowed the autopilot to continue the third approach where it did get back to the runway centerline prior to 1000 ft. In my opinion these excursions were fairly significant in that it took us over the centerline of the parallel taxiway. During parallel approaches this could possibly lead to loss of separation. Also; in IMC this could be quite disconcerting. These were all three separate aircraft on three separate days. I feel it necessary to investigate further. I've seen these on ILS and LOC approaches but always felt it was interference from aircraft in the ILS Zone. This being a RNAV; I don't have an explanation for this to happen at all much less repeatedly.