Hi

I am trying to do

awk -F: '{print $3}' a:b:c

awk: can't open a:b:c

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It's the expected behavior. If you really need to do something like this with AWK, you should write something like this:

awk 'BEGIN { 
  split(ARGV[1], t, ":")
  print t[3]
  }' a:b:c

For a string try this:

assuming: var="a:b:c"
echo "$var" | awk -F: '{print $3}'

More possibilities:

echo 'a:b:c' | sed 's/.*://'

echo 'a:b:c' | awk -F: '$0=$3'

Actually,
it could be easily done in shell without any external command
(unless, as already stated, there is another reason to use AWK in this particular case):

% s=a:b:c
% (IFS=:;set -- $s;printf "$3\n")
c

zsh-4.3.4% print ${${(s.:.)s}[3]}
c