Question

the average of three consecutive odd numbers is -3

Answer 1

The average of 3 numbers would be the sum of them divided by 3 so
(a+b+c)/3=-3 (eq. 1)
and knowing they are consecutive odd numbers
a=a (we don't know yet)
b=a+2 (+2 because b and c have to be odd)
c=b+2 so c=a+2+2

substituting eq.1
a+b+c=-9
a+(a+2)+(a+4)=-9
3a+6=-9
3a=-9-6=-15
a=-15/3=-5

so b = -5+2 = -3
and c = -5+4 = -1

proof
(-5-3-1)/3=-3