I watched the Parker Solar Probe launch live on NASA TV and, with respect Kevin, there is no way that you could have seen the launch at around 6pm local time, it actually occurred at around 5pm. Yes it had three Delta boosters strapped to it but it was launched from Florida and they were separated after boost phase, east of Florida over the Atlantic. You may have seen the relight of the second stage, after the coast phase, to boost the orbit, the timeline is about right for that. I believe the second stage engine was a single nozzle type, but thruster activity would be happening for course correction. Still, pretty hard to see thrusters firing from earth at that distance. You could watch the replay on NASA TV and maybe work out the timing overhead from that. Perhaps you saw the three boosters re-entering the atmo and burning up, but two would have likely come down long before the central one, which burned much longer.
https://www.nasa.gov/nasalive