How many Ensure Plus drinks is it safe to drink per day to gain weight?

Basically, I'm 17 years old and am looking to gain weight quickly without having too many side effects and health problems in the future. I heard these drinks contain a lot of sugar and diabetes is a problem if you drinks too many of them but I'm not sure. Btw, I consume about 1500 calories a day from normal food and take about 3 Ensure Plus's a day usually.