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.
point is both those CPUs are better for different things so you CANNOT recommend the i3 over the Pentium unless you know what he will be doing.
Also, EXACTLY what you just said applies to your previous comment about 2011 CPUs. They have different per-core performance, so your comment about clock speed is not only wrong, but incorrect fundamentally. Sorry.