Yes, VDDA3 is a voltage output. The voltage on VDDA3 is derived from the voltage on either VDD (1.8V to 3.6V), or VDDB (3.3V) if USB is connected. Furthermore, there is no internal regulation on VDDA3 (or VDDIO). These rails follow the voltage applied to VDD (when USB power is not applied).
For example, if a customer applies 2.5V to the VDD pin, then VDDA3 output will follow VDD (2.5V in this case) UNTIL USB is connected, applying 5V on the VBUS pin. At that point, the chip will be powered by the VDDB output (3.3V) and VDDA3 would then output 3.3V (following VDDB pin).