Showing a blank cell when no data is used. Or showing a blank cell formula unused

So i need a formula that gives me an answer, but, remains blank if the cells are giving the formula no data to compute. Please see below. This is how far i have gotten, but, i need to add something that says if no entries/no results:

=if(and(l13>3,m13>3,n13>3),"q","u")

THIS WORKS GREAT AS IT SHOWS ME TRUE/FALSE (OR Q/U)

BUT -

WHEN I HAVE BLANK ENTRIES IN THOSE 3 CELLS, IT SHOWS ME "U" WHEN I NEED IT TO SHOW NOTHING AT ALL.