I'm an 18 y/o college student eating 2-3 larger meals/day. i am 115 lbs with little muscle weight. how can i gain weight?

BMI vs body comp: You can use body mass index (bmi) to get starting point as to appropriate weight for height (regardless of gender or age). However, body composition (muscle vs fat) is also important consideration. Rather than focus on gaining weight (what if it's all fat?), focus on eating healthy eg mediterranean diet. And get some physical activity each day so that you'll gain muscle weight rather than fat weig.