The true voltage drop across the three resistors in series is divided in proportion their resistances.
Voltage across 3000 ohm resistor V,
When the voltmeter of resistance 6000 ohm is connected across 3000 ohm, the effective resistance of the 3000 ohm arm will be,
So the voltmeter reading will be:
Percentage error
Voltmeter reading will be 16.6 % lesser than the true voltage across 3000 ohm resistor.