There's been guides and videos about the mechanics and process for a while, so a quick search on YouTube should yield some decent results, saving me the time of typing up the whole essay of a how-to guide you'd bee looking for lol
However, I can answer your gen3 question. IV's do exist in gen 3, and are a part of the reason gen3 could not be backwards compatible with gen2, the way 2 was with 1. The IV's and EV's completely changed how a Pokemon's stats were calculated in the system, making the gen2 to gen3 translation very difficult to conceptualize. However, gen3's IV breeding and EV training alike would prove to be the most archaic, and naturally, outdated, by modern standards. In XY and ORAS, you can have parents hold a Destiny Knot to pass down 5 IV's to the offspring (selected at random from each parent), and an Everstone to pass down a certain nature to the egg, making it very easy to breed for competitive level Pokemon with just a little time and effort.
However, gen 3 has none of this. Or, if it does have certain features (I think the Everstone mechanic MIGHT have been put into Emerald, I don't entirely recall), it was only an increased chance at passing the nature down, and even then I think only the female could pass the nature. Other changes happened over time as well. Egg moves used to only pass from father to egg, but now both parents can contribute egg moves. There's probably other things I'm not remembering off hand, but that about covers some of the basic gen 3 to 6 changes in breeding mechanics.