0.84 tons= 1680 lbs
1 kg≈ 2.2 lbs
So........1680 lbs * 1 kg / 2.2 lbs = 763.63 kg
And.......using a proportion
x / 3kg = 1m^3 / 763.63 kg
763.63 kg * x = 3 kg * 1 m^3 [ cancel the kg ]
763.63 * x = 3 m^3 divide both sides by 763.63
x ≈ 0.0039 m^3 of soil
