a,b,yes
x,y,no

a,b,yes

res=yes
awk -F',' "($3 == "$res")" file.txt

awk: cmd. line:1: ( == yes)
awk: cmd. line:1:   ^ syntax error
awk: cmd. line:2: ( == yes)
awk: cmd. line:2:         ^ unexpected newline or end of string