It comes down to luck. 1 bell will always be super common while the 2nd bell will be common but not as common as the 1st bell. The 3rd bell is going to be somewhat rare to get. It's IGG's way of not simply handing over tons and tons of prizes. (while also filling up our inventory with junk :P)

You just need to keep killing mobs and pray you get some of the bells. Hope you find some ^^