Triple Slit Experiment

Yes. Three or more slits were predicted, tested and proven not to alter the results (unlike the two slit experiment result).

http://en.wikipedia.org/wiki/Double-slit_experiment

Has anyone been able to determine exactly where the state changes from particle to wave;is it within the slit itself,just after the slit,or just before?

It may be explained in the link above (or the links within the link above).

I had a quick look but couldn't see it.

All i know is that if the photon would slow down a scosh to think about it, well that would certainly change things... Milo

Not only has the problem been tested with multiple slits, there is an entire industry that exists due to this phenomenon. Hundreds of companies exist for making different types of diffraction gratings, including a range of devices from 'crude' Ronchi rulings to extremely fine holographic interference films. Gratings are the basis for thousands of optical devices (and toys), primarily replacing the glass prism for creating a spectrum from a beam of light. They are extensively used in spectroscopy and laser studies.

http://en.wikipedia.org/wiki/Diffraction_grating

The interference phenomenon has even been demonstrated with molecules (carbon nano-balls or 'buckyballs') large enough to be visible in microscopes.

This indicates to me that all matter may be of a dual nature.If not, where is the boundary or maximum particle size limit?

Yes, theoretically all matter is of dual nature.

I doubt that there is a boundary, per se. I think it's more a matter of signal to noise ratio. The test has been run with macro-sized particles but the results have been a simple summation of two single-slit diffraction patterns, not the interference pattern seen with nano particles (quanta). My guess is that if the test were performed with a wide range of 'particles' from electrons to buckyballs to large 'BB' sized pellets (and many sizes in between these) the interference pattern would gradually disappear from the results. Perhaps if the test could be run in true isolation, like at ultra cold temperatures and away from any outside 'macro' influences, the result for macro particles would be an interference pattern.