I've been running every day 1.7 miles to 3 miles, eating nothing but salads and healthy boring stuff around 1000 calories -- and yet I only fluctuate around 3 pounds difference... gaining it, and then losing it. This is making me nuts and super depressed, as I have been doing this for about 3 and a half months.

Can someone tell me what is going on?

Genetics and metabolism problems could be the root of that. But I'm sure there are more possible ways.

You can't just run, you have to mix up your routine. Your body will get used to running. You more or less have to "surprise your body." that's kind of what my trainer told me. And I'm not a health expert but maybe you need to boost your metabolism by eating small meals and eating more of them. That's what the lady that works at the gym told me.

Same goes for your calorie intake. Your metabolism adjusts to the amount of calories you eat a day. If you've been eating 1000 cal per day and not losing any weight, then your metabolism has adjusted to only burning that much.

Another possibility is that you think you are only eating 1000 cal per day when in reality you are eating more. I'm not sure how certain you are of your calorie intake but that could be something to consider as well.

Also, are you building muscle from your training? You may be burning fat but building muscle. Muscles actually weigh more than fat so the scale may be indicating that you are at the same weight but your body is more toned and you are in better shape regardless. Also, your weight is supposed to fluctuate depending on a number of factors which is why it's usually suggested not to weigh yourself everyday and not to worry about what the scale tells you too much.