Well, for example, let's compare the i7-5960X and the 5820K (I know they're not cross-platform, but whatever).
The 5960X has eight cores, but the 5820K has 6 (I believe). To compensate, the 5820K has a higher clock speed, and consequently, it's faster (at least in gaming). If the i3 has, say 2.8 GHz, the i3 kinda has a slight advantage on that.