First, it's no longer a rumor. The Coffee Lake i3s will be four core (and i5s six core). Second, you can just about guarantee the 8th gen i3s being more than their respective predecessors. How much more is anyone's guess because they have a direct competitor with Ryzen 3's four cores. Same with the motherboards.
I would wait until reviews of the i3 8100 are out and compare them to that of the $130 Ryzen 3 1300X which are already out.