As has been demonstrated previously, a dot after a note increases the value of the note by half (see page 12). A second dot (placed after the first) increases the value of the note by half the value of the first dot.
a dotted crotchet equals a crotchet plus a quaver.
a double dotted crotchet equals a crotchet plus a quaver plus a semi quaver.
This formula is the same for all note values.