harry potter - How exactly is Gringotts secure?

"Wizards have banks?"

"Just the one. Gringotts. Run by goblins."

Harry dropped the bit of sausage he was holding.

"Goblins?"

"Yeah -- so yeh'd be mad ter try an' rob it, I'll tell yeh that. Never mess with goblins, Harry. Gringotts is the safest place in the world fer anything yeh want ter keep safe -- 'cept maybe Hogwarts. As a matter o' fact, I gotta visit Gringotts anyway. Fer Dumbledore. Hogwarts business." Hagrid drew himself up proudly. "He usually gets me ter do important stuff fer him. Fetchin' you gettin' things from Gringotts -- knows he can trust me, see.

"Got everythin'? Come on, then."

_{Harry Potter and the Philosopher's Stone Pg 48 (emphasis mine)}

And then...

“I need to break into a Gringotts vault.”

Harry had not meant to say it so badly...

“Harry—” said Hermione, but she was cut off by Griphook.

“Break into a Gringotts vault?” repeated the goblin, wincing a little as he shifted his position upon the bed. “It is impossible.”

“No, it isn’t,” Ron contradicted him. “It’s been done.”

“Yeah,” said Harry. “The same day I first met you, Griphook. My birthday, seven years ago.”

_{Harry Potter and the Deathly Hallows Pg 410 (emphasis mine)}

And then......

“Never mind that stuff ... Is it true? Did you break into Gringotts? Did you escape on a dragon? It’s everywhere, everyone’s talking about it, Terry Boot got beaten up by Carrow for yelling about it in the Great Hall at dinner!”

“Yeah, it’s true,” said Harry.

_{Harry Potter and the Deathly Hallows Pg 480 (emphasis mine)}

If Harry, Ron, and Hermione were able to completely tamper the security of a super massively big bank holding billions of galleons, and other precious things, how could it be considered secure?

There could be many many such people, such attackers!

Answer

Griphook states that Quirrell succeeded because the vault had been emptied, and therefore the protections around the vault weren't at full capacity; it makes sense not to waste too many resources protecting an empty vault.

“The vault in question was empty at the time,” snapped the goblin, and Harry understood that even though Griphook had left Gringotts, he was offended at the idea of its defenses being breached. “Its protection was minimal.”
_{Harry Potter and the Deathly Hallows, Chapter 24 - The Wandmaker}

There's really very little known about how he got in and back out again, but we'll give Griphook the benefit of the doubt and assume that it was possible only because the vault was empty.

In the case of Harry, Ron and Hermione, they had the assistance of a Gringotts goblin. Griphook reacted angrily when Ron suggested he steal from the Lestrange's vault as payment, so I think it would be almost impossible for any other wizard to convince a goblin to help them; they'd have nothing to offer, and the goblin wouldn't believe - as Griphook did in Harry's case - that they weren't doing it purely for their own personal gain.

“Thief, you have been warned, beware — yeah, I know, I remember,” said Harry. “But I’m not trying to get myself any treasure, I’m not trying to take anything for personal gain. Can you believe that?”

The goblin looked slantwise at Harry, and the lightning scar on Harry’s forehead prickled, but he ignored it, refusing to acknowledge its pain or its invitation.

“If there was a wizard of whom I would believe that they did not seek personal gain,” said Griphook finally, “it would be you, Harry Potter. Goblins and elves are not used to the protection or the respect that you have shown this night. Not from wand-carriers.”
_{Harry Potter and the Deathly Hallows, Chapter 24 - The Wandmaker}

On top of Griphook's assistance, they had Polyjuice potion to transform into the owner of the vault, Bellatrix Lestrange, and also had identification to prove Bellatrix's identity in the form of her (old) wand, taken from her before escaping from Malfoy Manor.

Even with all of those advantages, they were forced to resort to use of an Unforgivable Curse to get to the vault, and then needed a dragon to escape after retrieving Hufflepuff's Cup from inside.

It was, quite simply, the most astounding set of circumstances that allowed them to succeed.