My answer if you are full f2p is to not get any of them. You'll eventually get sage naruto out of free draws from sage treasure, you'll get wb asuma from chases and survival and you'll get iruka by refining ninja fragments in excess.
So, hoard your coupons for a very rare ninja or just spend everything on powerups. The only cheap ninja worth to spend coupons on it right now is sakura sailor uniform.