The problem is that you require ' characters to delimit strings. Here is a workaround not using a here doc:
Code:
userid=me
pswd=mypassword
command=$(
echo "$userid/$pswd"
printf "insert into table1 (a1, a2, a3) values ('%s','%s','%s');\n" $a $b $c
echo "commit;"
echo "exit")
echo "$command" | sqlplus -s
