http://en.wikipedia.org/wiki/Hausdorff_dimension
Behaviour under unions and products
If
is a finite or
countable union, then

This can be verified directly from the definition.
If X and Y are metric spaces, then the Hausdorff
dimension of their product satisfies

An example in which the inequality is strict has been constructed
by J. M. Marstrand[2]. It is known that when X
and Y are Borel subsets of
,
the Hausdorff dimension of
is bounded from above
by the Hausdorff dimension of X
plus the upper packing dimension of Y. These facts are discussed in Mattila
(1995).
I don't think you
are going to like this:
two minimum dimensions are:
for projection dimensions:
s(x,y),s(y,z),s(z,x):
smin_1>=Min[s(x,y),s(y,z),s(z,x)]
or the second topological product:
smin_2>=s(x,y)*s(y,z)*s(z,x)
What it amounts to is the minimum dimension
of any of the projections rules the whole.
When determined in three dimensions the result may be larger
( probably is),
but with the projection measure you can only claim the minimum
as I understand it.
Say you have a plane with a dust scattered on it in s(x,y)=0.5.
looking at it edge wise it is a line broken in parts always less than
one
in the s(y,z) and s(z,x) directions,
so the dust 0.5 dimension "rules" the set.
The extreme case forms the limit for all the other cases in an
Hausdorff type dimension.
The result is the hard logical one:
an intersectional "and" of the Venn diagrams for dimension.
We would like to say
s=1+(s(x,y)-1+s(y,z)-1+s(z,x)-1)
but that is probably too big!
We have to think much like the Lyapunov coefficients: L(z,x) the
largest
s=1+L(x,y)/L(z,x)+L(y,z)/L(z,x)
If we treat your dimensions s(x,y),s(y,z),s(z,x) like Lyapunov
coefficients, then the "likely" or probable dimension is
s_likely=(s(x,y)+s(y,z)+s(z,x))/Max[s(x,y),s(y,z),s(z,x)]>=Min[s(x,y),s(y,z),s(z,x)]
I hope that helps.