The default leveling system of oblivion is pretty complicated, well at least to get those 5x every time on 3 different attributes. What you need to do to get a 5X on a specific attribute is leveling a corresponding skills 10 times. So if you want to level strength you can level the blade skill 10 times, 4 levels blade and 6 levels blunt or any combination of strength skills.
The problem is if you level 10 major skills you will level up so you have to pick the skills carefully when you create your characters. With carefully i also mean unlogically, since if you want to build i fighter and have block and heavy armor (both endurance skills) you can only get +5 to endurance if you level only those major skills, or mix in some armorer (also a endurance skill). So if you level block 3 times, heavy armor 3 times and another major skill like blade 4 times you will not get 5X to endurance unless you leveled armorer 4 times as well that level.
Since you are able to pick 3 5X each level up things get weird and complicated. What i usually did is pick a skill for every attribute and make it the least favorable skill for your character so you have more control on when you will level up. Since if you would pick light armor as a major and wearing light armor at the same time that skill would level up very fast and you will have to get rid of the armor or you will risk a level up without 5X picks.
In your case, if you would pick Alteration, conjuration and illusion as a major skill you could get 5x to all corresponding attributes like this:
- level mercantile/speechcraft 7 levels.
- level Alchemy/mysticism 7
- levels. level Destruction/restoration 6 levels.
You need to do all the above before you level up.
- Level illusion 3 levels.
- Level Conjuration 3 levels.
- Level Alteration 4 levels.
Only this way, or at least with combinations like this would get you 5X at all 3 attributes.
This however is called power leveling and will make you very strong later in the game so you do not need to play like this. In my honest opinion it wastes all the immersion of the game since you need to be constantly counting your levels.
However, there are mods that count this for you. There are also mods that modify the leveling system which i personally prefer. I used the "AF level mod" very often, it raises your attributes at the same time as you level the corresponding skills. You do not have to sleep either. You can just play the game without ever worrying about getting those 5X modifiers.