How to group by arraylist in c#.net

The best things to do are:
1) Stop using ArrayList - it was superseded by generic collections in V2 of .NET!
2) Create a class to hold the values, and give it a constructor which takes a string.

First, split each string using string.Split:
Then use DateTime.TryParseExact to convert each date to a DateTime value, and intTryParse to convert the ID:

private class MyData
            {
            private int _ID;
            public int ID
                {
                get { return _ID; }
                set { _ID = value; }
                }
            private DateTime _Date;
            public DateTime Date
                {
                get { return _Date; }
                set { _Date = value; }
                }
            private string _Type;
            public string Type
                {
                get { return _Type; }
                set { _Type = value; }
                }
            public MyData(string s)
                {
                string[] parts = s.Split(';');
                if (parts.Length != 3) throw new ArgumentException("Not a valid MyData element: " + s);
                if (!int.TryParse(parts[0], out _ID)) throw new ArgumentException("Bad ID Value: " + parts[0]);
                if (!DateTime.TryParseExact(parts[1], "yyyy-M-dd HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.None, out _Date)) throw new ArgumentException("Bad date format: " + parts[1]);
                _Type = parts[2];
                }
            }

Use each string to generate a new instance of MyData, and store them in a List<MyData>
Then it's pretty trivial to use a Linq query (or even a foreach loop) to get the totals grouped by date, ID, or both.