Kellogg's Rice Krispies!
Here they are in an advertisement from 1939. No Snap, Crackle and Pop (as characters) yet, back "Then":
But in 1950, there they are on the box:
And featured more prominently in this ad from 1953:
And the box "Now":
I must admit that, when I purchase them these days, it's usually because I want to make some of these (this ad from 1962):
And a Snap, Crackle, Pop to you!