A simple, although awk-less solution in bash:
while IFS=, read -r a a a b; do echo "$a"; done <inputfile
It works faster for small files (<100 lines) then awk as it uses less resources (avoids calling the expensive fork and execve system calls).
EDIT from Ed Morton (sorry for hi-jacking the answer, I don't know if there's a better way to address this):
To put to rest the myth that shell will run faster than awk for small files:
$ wc -l file
99 file
$ time while IFS=, read -r a a a b; do echo "$a"; done <file >/dev/null
real 0m0.016s
user 0m0.000s
sys 0m0.015s
$ time awk -F, '{print $3}' file >/dev/null
real 0m0.016s
user 0m0.000s
sys 0m0.015s
I expect if you get a REALY small enough file then you will see the shell script run in a fraction of a blink of an eye faster than the awk script but who cares?
And if you don't believe that it's harder to write robust shell scripts than awk scripts, look at this bug in the shell script you posted:
$ cat file
a,b,-e,d
$ cut -d, -f3 file
-e
$ awk -F, '{print $3}' file
-e
$ while IFS=, read -r a a a b; do echo "$a"; done <file
$