Passing variables into awk from bash

I am writing a shell script file in which I have to print certain columns of a file. So I try to use awk. The column numbers are calculated in the script. Nprop is a variable in a for loop, that changes from 1 to 8.

avg=1+3*$nprop
awk -v a=$avg '{print $a " " $a+1 " " $a+2}' $filename5 >> neig5.dat

I have tried the following also:

awk -v a=$avg '{print $a " " $(a+1) " " $(a+2) }' $filename5 >> neig5.dat

This results in printing the first three columns all the time.