Cree doesn't expect these COBs to be powered at less than 50W, I guess, so the chart starts at 1400mA . (50W = 1400mA x 35.7V) Looking at the green line, for a case temperature of 85 deg C, the voltage at 2100mA is 35.25. At 1400mA, the COB forward voltage is 34. So due to the electrical characteristics of the LEDs, the voltage goes down 1.25V when current is reduced by 700mA.

The forward voltage at Tc=85 with 700mA should be about 34v - 1.25V = 32.75V for each COB.

What the Voltage vs Current graphs show us is the importance of low case temperature. The lower the temperature, the higher the forward voltage and we get greater power at the same current (compared to a hot COB).

100% load on the driver would be LEDs with Vf adding up to 129V at 700mA. Proportionally, 33V x 3 = 99V and 99V / 129V = 77% load on the HLG80. This can be dimmed by 60% (420mA / 700mA) and 60% of 77% would be 46% load on the driver. Looking at the Efficiency vs Load graph, efficiency is 89% at the minimum circuit load (46%).