Bash: Creating a second stderr alternative

I have a bash script that is intended to have an alternative second stderr analogue:

exec 3>/dev/tty
echo "STDOUT" >&1
echo "STDERR" >&2
echo "STDDEV" >&3

Thus I’d like to filter out each of it’s streams differently. It works correctly in the next cases:

1. When I do not filter any stream.

$ bash test.bash 
STDOUT
STDERR
STDDEV

2. When I suppress stdout

$ bash test.bash > /dev/null
STDERR
STDDEV

3. When I suppress stderr

$ bash test.bash 2> /dev/null
STDOUT
STDDEV

Unfortunately, the attempt to suppress the stddev doesn’t results in actually suppressing it:

$ bash test.bash 3> /dev/null
STDOUT
STDERR
STDDEV

What am I doing wrong and how to fix it?
Thank you.

>Solution :

You could add a condition before creating FD3:

{ >&3; } 2>/dev/null || exec 3>/dev/tty

echo STDOUT >&1
echo STDERR >&2
echo STDDEV >&3

{ >&3; } evaluates to true when FD3 has been defined externally, like in test.bash 3> /dev/null