Motivation can go away the moment you change your mind. Don't rely on motivation.

Make a schedule for yourself and do it, even though you don't want to. You can stop setting yourself up for failure by following a self-imposed routine. Also, make a task list. Order it by priority. Do your tasks and space them out when appropriate. And by appropriate, it's not after you muck around on reddit. Do your tasks, then reddit or something. It'll become routine, but only if you stick to it. Don't put it off because you'll only be cursing yourself when your grades plummet again. Make room for your studies and homework in your schedule. I recommend you do your work right after your classes that same day before you return home. Drop by the school library or a similar place for no distractions.

Also the next time you head to bed at 3 in the morning, set your alarm clock and wake up by 9AM-10AM. You'll still be tired, but that is good because it means you'll be wanting to go to bed earlier. Stick to this pattern until you can actually sleep and wake up at a reasonable time and function normally. Restrict the time you're in bed as well. The moment you wake up, get out within 5-10 minutes and take a shower. Make your bed and don't crawl back into it until you intend to sleep for the night.

As for homework, if you're on your computer/laptop, stick to the sites that would help you do your work. Get off reddit. Also, install GetColdTurkey on your browser as it blocks your usual sites until you're done. If you don't need the internet to do your homework, turn off the wifi on your computer/laptop. Your grades are your priority and don't let yourself down.

As for looking for a job and doing gym stuff, do things one at a time as to not overwhelm yourself. Fix your sleeping pattern first, follow your schedule, then as it gets easier, include job hunting and hitting the gym. Job hunting is a beast, so you need rest and getting your stuff together.