  We present an analysis of the Minkowski Functionals (MFs) describing the WMAP
three-year temperature maps to place limits on possible levels of primordial
non-Gaussianity. In particular, we apply perturbative formulae for the MFs to
give constraints on the usual non-linear coupling constant fNL. The theoretical
predictions are found to agree with the MFs of simulated CMB maps including the
full effects of radiative transfer. The agreement is also very good even when
the simulation maps include various observational artifacts, including the
pixel window function, beam smearing, inhomogeneous noise and the survey mask.
We find accordingly that these analytical formulae can be applied directly to
observational measurements of fNL without relying on non-Gaussian simulations.
Considering the bin-to-bin covariance of the MFs in WMAP in a chi-square
analysis, we find that the primordial non-Gaussianity parameter is constrained
to lie in the range -70<fNL<91 at 95% C.L. using the Q+V+W co-added maps.
