I have noticed spellcasters ability to get bonus spells from having a high ability score depending on class (wisdom for clerics and paladins, intellgence for wizards, charisma for bards and sorcerers, I forgot about druids and rangers). Well, what I want to know is, is there a way for someone to get one of their scores up to 30? That includes temporary affects such as spells (ones that aren't permanent, but ones that are count too of course) and items of any type that could raise that one score. If you can find a away for any of them, that works, but it would be great for someone to find a way to do so for all three (I do know that the solutions would be similar, but different for each score). If you can't find a way for those scores to get up to 30 but you do have one for strength, dexterity, or constitution, that work too.
A starting score of 18, +2 bonus from race, 5 improvements as you level up (4th, 8th, 12th, 16th, and 20th), a +6 enhancement bonus from a magic item, and a +5 inherent bonus from wish or a manual/tome, results in a score of 36.
This is a fairly typical “all-in” ability score at 20th level. Depending on race, class, and the particular ability score you’re interested, you can do better. For example, an orc barbarian could have 46 Strength instead, because they get a +4 bonus from race, and mighty rage gives +8. But pretty much any class can get a 36 in any ability score, provided they push it as hard as they can.¹
You cannot, typically, afford to reach scores that high in more than one ability score. The 5 improvements from level can only be to one score, and wish or manual/tome are quite expensive, so one score at 36¹ and another at 20–24² (and the rest in the 8–14 range mostly) is a pretty typical end-result at 20th level.
A 34 may be more common, however, since 36 relies on that +2 from race, and many characters won’t have that because human is one of the most popular races in the game, and doesn’t get any +2 bonus. A 32 because you only started with 16 is also fairly common, since the point-buy cost of an 18 is quite high.
Depending on initial character creation, e.g. how much point-buy you had and how hard you were willing to dump everything else.
Though KRyan's answer is absolutely correct, I'd like to elaborate on having all three stats at 30. Just for completeness. It may be impractical but it is definitely not impossible. Magic items with +6 enhancement bonus would cost you 36,000 gp per stat (Periapt of Wisdom, Headband of Intellect and Cloak of Charisma), +5 inherent bonus would be either 25,000 XP per stat (5 consecutive wishes) or 137,500 gp per tome (Tome of Clear Thought, Tome of Leadership and Influence or Tome of Understanding wondrous items). Thus you have +11 total to each stat. It means you need only 19 in each stat at the start.
A 17 in a stat plus racial bonus can give you 19 in one of them. The rest you'll need to raise through bonus points at levelling up. 5 points at level 20 will allow you to start with 18 and 15 or 17 and 16. Or alternatively you can raise a 16 stat to 18 with racial bonus and use one bonus point on top of that, then you'll need other two stats at 17 and 17 or 18 and 16 initially. If you happen to have two stats at 18 (exceptionally lucky roll if you roll stats) you can have the third one as low as 14 to spend 3 bonus points and +2 racial bonus to get it to the required 19.
To sum it up you need stats at 17,17,16 or 18,17,15 or 18,16,16 or 18,18,14 before racial adjustment and either spend 520,500 gp or 108,000gp and 75,000 XP total. Since you are expected to have 760,000 gp by level 20 (table 5-1, Character wealth by level, PHB p.135), it is not cheap but affordable.