Arduino Due analogRead velues are quite unstable

Hi guys, I'm trying to read signal using Arduino Due,

setup is pretty simple (Signal 0.3-3V ) connected to A0
I set ADC to 12 bit,
and then just do analogRead(A0)
values for 0.3 V can be all over the shop ( 950-1050, and for 3V 3350-3500 )
While the multimeter reads 3.049V every time on the same lead.
Curious what is the problem