Looking from the top we can draw an equilateral
triangle of side length 2a connecting the
centres of the bottom spheres. The distance
to the centre, l, can be found using geometry
and is 2sqrt3*a/3. From a side view we can
see the forces acting on the top sphere. 
It's weight acting down is balanced by the 3 reaction
forces from the 3 lower spheres. On the diagram
the back sphere can't be seen. Equating these
gives 3Rsintheta = mg. Returning to an aerial
view we can draw on the tensions, T, from
the welds. These are balanced by the component
of the normal force, R, in the horizontal
plane, which looking at the above diagram
is R cos theta. Equating these will give a
value for T.
