Now considering just one of the table legs.
The table legs have a mass M, therefore there
is a weight Mg acting downwards. There is
also the Newton Three pair, R, acting downwards
from the tabletop. Finally there is a reaction
force, N, from the ground which keeps the
table leg in equilibrium. Therefore, using
Newton's First Law, the Forces must be balanced
on this object and resolving vertically gives us  N = R + Mg
