You can find my recent comments regarding the Melbourne Marriott here. I cannot comment on the Sheraton at all, as I don't recall so much as laying eyes on it (although my recollection from my own research is that it wasn't a big hit with those who've stayed there). As to the Westin, it's right in the thick of things, which depending on perspective can be a good thing or not (e.g. shorter walks but likely more noise). Looked like many rooms had balconies, although I never saw anyone making use of them (the weather was fair most days we were there). I did not, however, step inside to have a look about (which is odd for me, as checking out hotels in a new city is kind of a thing with me).

I'm sure you will have a lovely time in MEL regardless of your choice - enjoy!