I just broke my 3rd temporary crown in 4 days. Could it be the mix he is using to make the temporary is bad?

The reason the temporary keeps breaking is because it is too thin. Adjusting the opposing tooth is a common practice but if it keeps breaking I would suggest reprepping the tooth to create enough space for the temp. The real crown will fracture also if it is too thin. This may add some additional time and another step but it will be worth it in the long term.

I agree with Dr.Borowski...it breaks because the temporary is too thin which means that there is minimal or not enough reduction on the tooth itself. If the crown is going to be gold, it may be OK as it only needs a millimeter of thickness. Porcelain needs more thickness so the tooth may need to reprepared and more taken off or the opposing tooth may need to be reduced more if that is acceptable. Often the upper molars have "plunging cusps" which are pointed and need to be rounded off but if he already did this, then something else needs to be done. Be sure you aren't eating hard things,too