import (
    "gopkg.in/mgo.v2"
    "crypto/tls"
    "net"
)

tlsConfig := &tls.Config{}

dialInfo := &mgo.DialInfo{
    Addrs: []string{"prefix1.mongodb.net:27017",
                    "prefix2.mongodb.net:27017",
                    "prefix3.mongodb.net:27017"},
    Database: "authDatabaseName",
    Username: "user",
    Password: "pass",
}
dialInfo.DialServer = func(addr *mgo.ServerAddr) (net.Conn, error) {
    conn, err := tls.Dial("tcp", addr.String(), tlsConfig)
    return conn, err
}
session, err := mgo.DialWithInfo(dialInfo)

Addrs: []string{"prefix2.mongodb.net:27017"}

//URI without ssl=true
var mongoURI = "mongodb://username:password@prefix1.mongodb.net,prefix2.mongodb.net,prefix3.mongodb.net/dbName?replicaSet=replName&authSource=admin"

dialInfo, err := mgo.ParseURL(mongoURI)

//Below part is similar to above.
tlsConfig := &tls.Config{}
dialInfo.DialServer = func(addr *mgo.ServerAddr) (net.Conn, error) {
    conn, err := tls.Dial("tcp", addr.String(), tlsConfig)
    return conn, err
}
session, _ := mgo.DialWithInfo(dialInfo)