MC34063 Output Voltage Sagging
A few major issues with your specs.
The datasheet does not show it can boost 3V3 to 12V and output 1.5 for both conditions.
Looking at the Darlington saturation voltage of 1.5V and 3.3V input drops the input boost voltage of 1.8V so the input boost current is 12.5/1.8 almost an Amp (even if you cut all the other losses)
Your diode drop may also be excessive.
You ought to lower f for this chip which is over it's recommended operation due to the Darlington saturation recovery times even though for some uses it works.
Being a low cost may not be a good reason to choose this IC when there are far better ones using TI's Power Designer or similar.
DCDC design is hard and you should follow the TI design guide EXACTLY in every detail.  (This old chip needs a lot of work)