MyContatiner.MyArray(15,3)
1 -[N] - .top
2 -[N] - .left
3 -[L] - assigned

MyContainer.Init

*!* Step Through the controls (if you have more controls than the buttons, you may need to alter this)
FOR lnObjCount = 1 TO THIS.ControlCount
  lnRow = INT(RAND(0)*15)  && randomly select row 
  DO WHILE THIS.MyArray(lnRow,3)
    *!*  since row has already been assigned, randomly select another row 
    lnRow = INT(RAND(0)*15)
  LOOP
  *!*  set properties for buttons
  THIS.Controls(lnObjCount).Top = THIS.MyArray(lnRow,1)
  THIS.Controls(lnObjCount).Left = THIS.MyArray(lnRow,2)
  THIS.MyArray(lnRow,3) = .T.
ENDFOR

THIS.Visible = .T.
THIS.Refresh

Bear in mind, that this is quite simple and the DO LOOP could potentially drag on when attempting to assign the last row or two.

You could also set up the coordinates in a table and assign an index key at run time. This would ensure that it would be quicker.

HTH