We could play any class and have an excellent chance of a successful adventuring career. Heck, why not just gift the player a new Vorpal sword and +5 plate armor too, just to get the party started.
However, a little adversity makes the game more interesting.
On the other hand, no one has ever played a PC with primary stats of all 3s. Since you (and I) haven't, let's take a look at what this character looks like.
Name: Larry the Loser Str: 3 Int: 3 Wis: 3
Class: Fighter Dex: 3 Con: 3 Cha: 3
AC: 8 (chain mail)
DMG: 1d8 (-3)
THAC0: 20 (but really 22)
Wow, that's rough. Let's start with the hit points. A fighter gets 1d8 hit points at level 1. The expected value of that roll is 4.5, which I'll generously round up to 5. With a 3 constitution, I subtract 3 points from that. Larry starts off in the world with but the two hit points.
Next, I equip him with a normal sword and chain mail, which is a pretty average way to kit out a fighter. Chain mail normally bestows an armor class of 5, but "all thumbs" here must subtract 3 points. This makes his AC 8. Put another way, wearing leather or studded leather armor gives NO additional
protection to Larry.
No matter how well-made that sword is, on its best days, it is only 25% more effective than a dagger in Larry's hands. About 38% of the time when Larry manages to hit something, he fails to do any damage at all. He will be 5th when his THAC0 drops to 19.
He has no friends, is easily duped and barely verbal.
Let's all agree that a 3 in any primary stat is playable only for laughs.
So what is the reasonable spread of stats? The answer to this question is inherently subjective. Let's start off with some fuzzy numbers:
primary stat: very good
secondary stat: good to very good
tertiary stat: average to good
A primary stat is that primary ability that most impacts the performance of the class. For fighters, this is strength. For clerics, it is wisom. For magic-users, this is intelligence. For thieves, it is dexterity. It gets a little fuzzy for the demi-humans, particularly halflings.
The secondary stat is one that supports the primary. In most cases, this is going to be either constitution or dexterity, since these affect combat. In the case of a cleric, strength might be the secondary.
The tertiary stat is often something that is useful outside of combat, like wisdom or charisma. There is a lot of room for further discussion here.
A "very good" stat is 16 to 18. If your PCs highest attribute is a lukewarm 15, your character will have trouble standing out, I think.
A "good" stat is 13-15. This usually nets you a nice bonus for a non-essential stat and that's a good thing.
An "average" stats is exactly what you think: 9-12. No bonuses, but no penalties either.
What about the other three stats? These can be anything higher than 5. The 6-8 range provides lots of opportunities for interesting problems and role playing.
Less than 5 in a stat really suggests that a character isn't well-suited for adventuring at all. Even a magic-user with 3 strength will be hard-pressed to succeed. Think you can just move that 3 to charisma an be safe? Perhaps, but you're rude/ugly/unlikeable PC will certainly turn heads in a bad way, probably at the wrong time.
Next time you get a sub-par stat, embrace it for his "characterful" opportunities.
And now, the Shambling Mound: