Extreme4 is a lot better in terms of overclocking&stability since Extreme3 has those ancient D-PAK mosfets that are not reliable at all for modern power designs. Extreme4, on the other hand, has Low RDS(on) Renesas or NXP mosfets. Also has more complex power phases (6 hybrid/analog) than Extreme3 (4 hybrid/analog).
Even asrock Z87-Pro4 is better than Extreme3 since it has same mosfets with Extreme4 just with less complex 3 true power phase and less capacitance. But still better than Extreme3.
So I wouldn't recommend Extreme3 as there are a lot more better boards on the market in any day. None of the manufacturers are using D-PAK mosfets for years even in their low-end boards. It's weird to see that ASRock still uses them often.
I don't know what are your other options with 100€ though since prices change alot from region to region. If Extreme4 is in the range, or not dramatically over it, then buy that board. Otherwise check for Gigabyte or MSI z87 solutions.