We know that g=GM/R^2
Mass of earth and G are both taken as constants.
so g is inversely related to R^2 (R=radius of earth)
1.So as R is more at equator g is less but vice verse in poles.
2.if we ascend to higher places then we will increase the distance from us to the center of earth so the new R'=R + the height attained by us, so again more R so g will be less  
3. if go underground also g is affected.
Thanks for your answer
If we go under ground also g decreases / g=GM/R^2 in depth g'=GM'/(R-h)^2 but as mass is not same as mass of earth g'< g considering the reduced mass