make a container that can only hold 3 kg of water, and the wood block can be put inside from the top open.
now add 3 kg of water in the container, and put the wood block in.
if part of the wood block stays up the container open, then the water remaining in the container is 1 kg.
According to http://www.engineeringtoolbox.com/wood-density-d_40.html, only a few wood is heavier than water, so most wood will 'float'.