apply formula to selected cells via macro

[Axe]

i've been struggling unsuccessfully, any help would be appreciated: trying to set up a macro to convert a range of user-highlighted(selected) cells to 3 significant figures:

for example, convert 0.135564 to 0.136

the equation i found elsewhere online (and humble thanks to the original author, whose name escapes me):
ROUND(xx,3-(1+INT(LOG10(ABS(xx)))))

but i can't quite figue out how to apply the equation to a selected range of cells via a macro... , any/all help greatly appreciated!

thanks,
-axe