Is a 3.3V-5V MCU runnable at "nominal" 3.3V?
Not really, at least not in your case.

This MCU comes in two variants: One designed to operate on a nominal 5 volt, and one designed to operate on a nominal 3.3 volt.

The datasheet is just a little bit odd, but you can draw some conclusions from a few places. Chapter 1, section 1.1: Features:


  Operating voltage range: 5.5V ~ 3.5V or 2.2V ~ 3.6


Clearly, they think that the 5 volt variant is good between 3.5 to 5.5 volts. If they knew that it worked well for a lower voltage, they would have included it in the features. It's a selling point.

Let's look at page 423 which you linked to:


  DC Specification (5V MCU)
  
  VDD   Operating Voltage   Min: 3.3  Typ 5.0  Max: 5.5


Now, here the range is wider. Why? Not sure. There are usually some test conditions. The common way to interpret these figures is that they guarantee by design and/or testing that the MCU will operate correctly for the whole temperature range and clock frequencies if you keep the voltage between 3.3 and 5.5 volts. Anything else and you're operating on luck.

Now, look at page 54:


  2.3.5 Power-On Reset (POR)
  
  When VCC drops below the detection threshold of POR circuit, all of the logic circuits are reset.
  The nominal POR detection threshold is around 1.9V for 3V device and 3.3V for 5V device.


There you have it. If you try to operate this on a nominal 3.3 volt, and it goes slightly below that for various reasons, your MCU will likely reset.