Merge two DataTables by adding corresponding entries
You are not testing for same size.

You don't use combinedDT34.  You are changing an input which is not a good practice.  I am not sure if dt3 will be passed by reference.

You are starting column on 1.   

static DataTable CombineDT34(DataTable dt3, DataTable dt4) 
{
    if(dt3.Rows.Count != dt4.Rows.Count)
        throw new ArgumentOutOfRangeException);
    //...

    for (var r3 = 0; r3 &lt; dt3.Rows.Count; r3++) 
    {
        for(var c3 = 0; c3 &lt; dt3.Columns.Count; c3++) 
        {
            dt3.Rows[r3][c3] = ( Convert.ToInt32(dt3.Rows[r3][c3]) +
                                 Convert.ToInt32(dt4.Rows[r3][c3]) );
        }
    }
    return dt3;
}