Be careful with sports drinks, the might help you gain weight but they contain either a lot of fat and sugar or harmful sugar substitutes and lots of artificial ingredients that can be harmful as well. Eating added protein in your meals and weight training will add muscle, but if you just want to gain any kind of weight eat a lot of simple carbs. They will turn quickly to sugar and add fat to your body and are unhealthy, but you will gain weight. Same with eating fatty foods, though fatty fish won't make you gain weight, it might even help you lose weight by increasing the efficiency of your body ridding itself of harmful sticky stuff that isn't digested and gets stored. The only supplements I know of that can help you gain weight would be those that slow your metabolism so you gain fat, such as any relaxant taken frequently. So in my opinion, the best way to gain weight healthfully is to eat more meals, as you're doing, but eat healthy food and add weight training to your exercise regimen so what you add is muscle, not fat.
The "American Journal of Clinical Nutrition" suggests that omega-3 fatty acids can and fish oil can also help you gain weight, as such supplements are calorie-dense due to the fat content and also promote increased muscle protein synthesis, which promotes muscle building.
After exercise, consume a high-calorie, high-carbohydrate recovery supplement. These supplements come in powdered form and can be mixed with water, but mixing the powder with milk or juice will increase the calorie content, which can help you gain weight more efficiently. Choosing a supplement with a 3:1 or 4:1 ratio of carbohydrates to protein may help promote optimal recovery, as protein can help build and repair muscle tissue, leading to weight and muscle gain.
Vitamins C and E may help. The health benefits when consumed individually, some research suggests that combining the two supplements may help you gain weight. A study published in the January 2010 issue of the journal "Diabetes Research and Clinical Practice" found that taking vitamin C and E supplements in conjunction with weight training produced greater increases in lean muscle tissue than weight training alone. These vitamins are considered antioxidants, as they can protect your cells from free radical damage, an effect that may prompt superior exercise recovery and growth.
Vitamin D helps in maintaining bone density, that can help prevent you from losing weight, while another effect of vitamin D can help you gain weight. A study published in the March 2011 issue of the journal "Hormone and Metabolic Research" found that increasing vitamin D intake led to increased testosterone levels. Testosterone is a key anabolic, or muscle-building, hormone, and it can aid in weight gain.