hmm, is 3rd shift like 2-10pm, or overnights? I would say try your best to plan ahead, becasue if youre tired or you don't bring a lunch to work it could be really tricky to try to find something legal..
an easy thing to bring to work would be low carb chili, and very satisfying too.
Or you could have a big salad with bagged mix, grape tomatoes, a bit of shredded cheese and diced sandwich meat (carb free and not too high fat) thats one of the fastest things to slap together for lunch. Then just some dressing in a tupperware mini.
Good luck! I have worked 2-10 AND overnights and both are doable,