Wellington Management Group LLP more than tripled its stake in Massachusetts companies American Tower Corp. and Repligen Corp. in the first quarter of 2015. The Boston-based investment management firm increased the number of shares it holds in Boston wireless tower communication operator American Tower by 218 percent to 6.7 million, valued at $634 million, according to a recent securities filing detailing its holdings as of March 31. It boosted its shares in Waltham drugmaker Repligen by 205 percent to 36,637, valued at $1.1 million.