Generating three voltages with a MCU
Your calculations are then incorrect. 

The ATmega644 works fine with 5 V (which is also close enough to the output voltage of the microcontroller which itself is rated at minimal 4.2 V on page 316 of the datasheet) and at that voltage using a \$ 330 \mbox{ } \Omega \$ and \$ 75 \mbox{ } \Omega\$ voltage divider, you get 0.926 V which is close enough to 1 V for most purposes. 

Using the \$1 \mbox{ } k\Omega\$ and \$ 75 \mbox{ } \Omega\$ voltage divider, you get around 0.349 V which is for most purposes close enough to 0.3 V. And when both pins are off, you get 0 V.