onfail is necessary only in very rare cases. Like if what follow foo_no_arg_ would be something that destroys the math expression, but at the same time corresponds to some weird filename.
Like:
$ gmic foo_noarg_ ").jpeg"
it’s unlikely to happen in practice, but who knows ? 
And you your other request, I’d propose something like this:
foo : skip ${1=?},${2=?},${3=?}
l[]
a,b,c=?,?,?
if ("(isnum($1) || ['$1']=='?') &&
(isnum($2) || ['$2']=='?') &&
(isnum($3) || ['$3']=='?')")
a,b,c=${1-3}
else noarg
fi
onfail noarg
endl
sp $a sp $b sp $c