They all work. To my ear, 'Bend the product by applying a force' sounds best.
Without context, it is difficult to advise, but even sweeter sounding is:
'Bend the product by applying force' (not no 'a')
However, this does conjour up an image of bending it over your knee.
'Bending of product by application of a force.' looks like a paragraph title. Again, you could remove the word 'a' from this for a more native-English feel.
All are grammatically sound.