Replacing Characters

[JB]

I want to replace every third character in
a text string if it is a certain character.  So
lets say I have a list of characters there
range from A thru F and the list is possibly
10,000 characters long.

I want to check every third character in the
list and if it is a D then I want to change it
to a +.

I can probably do it with a repeat and use a
variable for a counter that resets every three
times and then check the character when the
counter gets to three but I was wondering if it
could be done faster somehow.

John Balgenorth