You need to unmount the ext3 filesystem in order to shrink it.
umount -l means that the filesystem will be unmounted when there is no more open file on it.
lsof /tmp to see what files are open on
/tmp. If you're running an X server, you'll see its socket
/tmp/.X11-unix/X0. You can't remove that socket¹ and still be able to connect to the X server. Other than that, most files tend to be short-lived or to belong to programs that can be restarted.
If you really want to perform the operation on a live system, you'll need to migrate to a different
/tmp filesystem, at least for the duration of the operation. You could transition to tmpfs, in which
/tmp is stored in RAM or swap; see this guide. In fact, tmpfs for
/tmp is a common setup; you might want to stick to that and remove the
/tmp partition altogether (and perhaps enlarge your swap a little instead).
Once you've been able to close everything from
/tmp, you'll be able to unmount it. Don't use
umount -l, it's useless here since it frees the mount point but not the device, whereas what you want is to free the device. Once
/tmp is unmounted, run
resize2fs to shrink it, and shrink the LVM logical volume accordingly. Or you might in fact save time by directly shrinking the LVM volume and creating a new filesystem for
/tmp. (If you have any data you want to keep in
/tmp, you're doing it wrong.
/tmp is for data that need not be saved between reboots, and closed files in
/tmp are fair game for deletion.)
If all this seems daunting, reboot to a recovery system (live CD or USB) and operate from there.
¹ Nor can you move it to another filesystem: that would be removing the original and creating a new socket.