What, if anything, do other E3s of any other branches do for money as side jobs?

WOW! What a "Loaded-Question"! Should be interesting to see the responses; time to get the Pop-Corn ready for the show-LOL!

Actually, if you are an E-3 without a family/spouse, you should be able to survive just fine on your pay. You are living in the dorm/barracks & you have a meal card for 3-4 squares a day. Anything you make is actually disporable income. I can tell you that when I was a junior 0-3, there were many airmen that had better cars and certainly better sound systems than I had. My disposable income after paying for food, mortgage heat, cable and a family of 4 was about $150/mo. Depending on your MOS, I would suggest Pizza delivery, working at an auto parts store, seasonal work such as UPS delivery, telephone solicitation, hunting guide, service industry work, auto/boat/motorcycle/aircraft maintenance depending on your skill level. The economy is booming and there should be plenty of work out there. If you live in the "hinterland", talk to local farmers/ranchers and see if they need day labor to repair fences, carry hay, clean pens, work on tractors etc. Good luck.

-photography
-audio production
-fiver

Find something you have a skill or passion for. All work and no play makes you old and grey.

Just make sure it doesn't interfere with your other duties and is approved by the command.

I was a cook. I did mechanical work on enginemens vehicles at the base hobby shop.

Personally, I’m a writer on the side

After reviewing some of the comments I have to say open your own business. You have to keep in mind "your time" is still the military's time. If you open your own business, your time is flexible. I personally opened my own yard landscaping and maintenance business. It didn't require much of an investment to get started and the business transferred when I did. Of course I let my command know and it never interfered with my duty. I would advise against standing extra duty for cash due to the burn out factor and in most cases it is frowned upon by commands.

I worked at the NCO Club calling bingo, and at the enlisted club as a master at arms, barback, and bartender. It fell under Nonappropriated Funds (NAF).

Sorry. I'm old school.
By comparison, E3's of today are a member of the ultra-stinkin-rich and have no excuse for complaining about lack of money. Try living on $110 per month (before taxes) for a few years and learn to manage your finances

Teach E-2’s the ropes and technical advisor to new LT’s and Ensigns.

Smiling when I saw this question. When I was an E-3 in the early 80's. I worked in a shoe store. So #AlBundy