I flew chase for those flights, back in the day. Same config, one shot is from below, so the rotation appears backward.
The 3 bladed flights were flown to confirm the behavior of the rotor at lower blade loadings, so that performance for the next gen of Sikorsky helos could be examined. The ability to recover was also tested, of course, but at the cost of flight test, that was not the biggest driver.
The vibrations were simply awful, BTW. The normal 53 needs no vibration suppression gear, but since vibes go up when number of blades go down, the three-bladed 53 was another world. The pilots (Jim Kay and Al King) had difficulty reading the instrument panel.