Wow, so how did you get all that muscle if you never exercise? You must have some activity that put that muscle on. That's a good thing, I have floppy, flabby arms, so I'm insanely jealous.
So, you probably don't want to hear this, but not changing your eating habits and only working out 3 times a week isn't really going to do much for you. Weight loss/control requires both healthy eating habits and exercise. Interesting tidbit- compared to 40 years ago, our nation is massively, horribly fat. Yet compared to 40 years ago, we exercise a massive amount more. Why isn't it the other way? So much of what we believe about exercise and how it effects us is wrong.
But more importantly, what we eat is so screwed up it's ridiculous. Between out of whack portion ideas and the fact that we put sugar in literally everything, it's no wonder. So even if you're parents won't change what they buy, and you can't buy your own items, it would be way worth the effort to learn more about what's in your food, and what a correct portion is. I was way in denial about all this till recently, so trust me, it's ALWAYS better to read the labels than make an assumption.
As far as exercise goes, they recently learned that if you sit for a long period of time without getting up and moving around, "fat burning" nearly completely stops. Just getting up and walking around the room makes a HUGE difference. Relatedly, while 3 times a week is a good start, most people recommend at least half an hour of exercise every day. It's not about just the calories directly burned, but also that it keeps up your metabolism. Also, "exercise" doesn't always mean "cardio." It can mean walking, or whatever you enjoy. Maybe that will make it easier for you?
Sorry for the ramble there, I read about this stuff all day long. (Mostly because I'm bored at work and there's not much else to do.
) One of the main things I've learned is that people disagree about everything. So you have to research on your own and decide what makes sense for YOU. But hopefully the people around here will help.