Take a look to Voltage Divider Example.
There is everything you need.
10k and 3k3 doesn't give you 1:3 divider. Resistor ratio in your case should be 1:2 (e.g 11k and 22k).
Link above explain also divider math to calculate right values.
EDIT: Earlier post 12bit (4095) should have been 10bit (1023). My misstake!
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