ok, its something like
-0--0--0--0--0--0--0
-0--0--0--0--0--0--0
-0--0--0--0--0--0--0
-0--0--0--0--0--0--0
all the 0's are the spawnpoints, which are instances of an object (a sprite for example)
you have to Pick Random Instance of Sprout Point, and when you do, destroy the SproutPoint
then, when you destroy the tree, create a SproutPoint where it was
when you destroy the three, set a global Variable with the numbers of threes you've destroyed
Tree ~double click then
Create Objet Sprout Point tree.x tree.y tree.layer
destroy tree
set TreesToSprout = TreesToSprout +1
every N ticks (where N is the numer of time elapsed)
pick random SproutPoint
create object Tree — SproutPoint.Y SproutPoint.layer
destroy SproutPoint
set TreesToSprout = TreesTo Sprout -1
its the most simple i can explain, hope you got it