Not sure what you mean with the "+5 bonus AND a +3 bonus" comment...?
It's pretty easy to "work the system", if that's what you want to do. For example, to get +5 to your Int every level up, just make sure you've raised Alchemy, Conjuration and Mysticism a total of 10 points (in any combination). Of course, to do this, you have to be very careful about which skills you level, and in which order. Those reset every time you level, so if you level once, you have to raise those skills 10 points again at that point, if you raised them 20 to begin with, and then leveled, you're wasting 10 of the points.
If you're interested in just modding that so that you always get +5, it's pretty easy. Download the Construction Kit, and run it. Go to GamePlay->Settings. Scroll to the variables iLevelUp01Mult->iLevelUp10Mult. Those 10 variables control the bonus that you get depending how much you've leveled up your skills. Change them all to 5, and you'll get +5 all the time (well, you still need at least 1 point, because you can't change it for None). Then, save that, and it'll make an .esp file for you. Next time you run Oblivion, select it in 'Data Files'. Note, this only works for new games, it won't affect old ones (I think, I might be wrong on that, actually).