New oven/microwave combination unit with 4 wire pigtail
It's possible the newest units are simply not made for a 3-prong receptacle (and that would be reasonable since grounds are important to protect electronics from people).  

In that case, and regardless for safety, you have the ability to retrofit ground.   This involves running a #10 bare wire via any achievable route back to any of several locations:


the service panel the power comes out of 
any other appliance or junction which has a #10 ground wire or larger going back to that same panel 
the grounding electrode wire that grounds the service panel to your water pipe or ground rods