Well I'm not familiar at all using alsa as I'm lazy, but I guess one solution would be to run 'sudo amixer -c 3' automatically when you log in; place the script.sh in /etc/profile and add the /path/to/script.sh to the /etc/sudoers file so it can run without a password.
To do the script idea though, I guess I would need to dive head first into the systemD script structure. I tried once before but got a bit lost/overwhelmed by the spaghetti network of scripts. I wasn't sure where the proper place would be to insert my own.
They are called unit. Unit files are loaded from two locations. From lowest to highest precedence they are: - /usr/lib/systemd/system/: units provided by installed packages - /etc/systemd/system/: units installed by the system administrator
That being said, here you can find documentation on how to write systemd unit files.
Deb 8 comes with sudo out of the box, shell scripts have nothing to do with systemd, pulseaudio doesn't 'replace' alsa and running commands with sudo is more limited (read: secure) than using actual root... or maybe I got the worst case of alzheimers ever.
You might also consider pulseaudio if you remove the DAC now and then or use miltiple sources, it handles things better than ALSA, that said.. Both can be a pain to configure, if you need to configure them. I've had a good time with pulse the past couple years. But I also dont really use window managers, so sometimes less on the stack is better.
@Baz is correct though on the audio, pulse doesnt replace ALSA, its similar to Jack it uses ALSA to talk to the hardware.