"I need keep my RHI = 4.2v and RLO = 3."
There's only the battery voltage of 3V - 4.2V across each LM3914, so RHI has to be always a bit lower than 3V.
Due to variations in the 1.28V Ref, the internal 12k divider, and the ADJ current, I think you'd have to do something like that where you adjust VR1 for an accurate 2.1V on RHI, and not worry if the 1.5V on RLO is off a bit.
... or something.