$ awk 'function out(){for(n=1;n<=c;n++)print a[n]"\t"s/c;s=c=0}{if(S7!=$7)out();s+=$3;a[++c]=$0;S7=$7}END{out()}' FILE
a b 3 q e w aaa 3
a b 4 q e w bbb 4.5
a b 5 q e w bbb 4.5
a b 5 q e w ccc 5
a b 5 q e w ccc 5
a b 4 q e w aaa 6
a b 8 q e w aaa 6
a b 2 q e w ccc 3.5
a b 5 q e w ccc 3.5