mower blades round hole ,star hole?

bought a new set of mower blades for my cub 1042
They sent me the right size ,the right hole dia. and the right spacing of the 3 holes.
But,the center hole is round,whereas my old blades have a star shape hole.Is there any way I can use them ?
I was thinking maybe a lock washer.
Sending them back is not an option.
Thank you