/bin/sh: How to redirect from &3 to named pipe?

I have a background process read from named pipe. for example

mkfifo /tmp/log.pipe

./myprog.sh < /tmp/log.pipe

I want to use &3 instead of开发者_如何学运维 specify /tmp/log.pipe

echo "aaa" >&3 but results similar to echo "aaa" > /tmp/log.pipe

How can I redirect &3 to /tmp/log.pipe everytime.

exec 3>/tmp/log.pipe