that the three fillies in the Friendship statue would be turned to stone?

They were obviously plotting a coup.

They hoped to topple Celestia and erect a false democracy that they would rule from the shadows, but the truth is that their efforts were all a contingency plan set up by Nightmare Moon in case she were defeated, to remove her sister from power before her release.

However, this idea was secretly planted in her head by none other than King Sombra, a confidante of the throne from before he turned to evil.

Unbeknownst to Sombra however, is the truth that it was Celestia who used mind control to bend his will towards planting the message, as Celestia already knew Luna was turning to darkness and so decided to route any other traitors to the crown at the same time.

An action she would never have taken had Discord not placed a seed of paranoia in her mind long ago, in a long winding plan to cause Celestia's fall, ensuring Discord could rule forever.

But even Discord was but a pawn to the true conspirator, Princess Luna. Who used her dream magic to plant the idea of making Celestia's fall through paranoia into Discord's dreams, and engineering her elder sister's eventual downfall.

Or they ate the last cookie, not sure which.

Leukemia.

Discord's children?

[Stories in Stone reasoning.]

the three foals were infact Discord's children. the trio were created along with many other things by Discord because it amused him and furthered his own agendas. The trio were given similar powers to discord and turned loose.
Twilido
Twiliray
and Twilimee [later known as Screwball]

The trio were the reason that the former wielder of the powers of night and day was slain and Do and Ray took the powers causeing insane amounts of havoc as they played witht he sun and the moon.

Celestia, Luna, and,the other Element Bearers of the time came across them and tricked the trio and sealed them in stone. When they were defeated the stolen powers were passed on to Luna and Celestia as they were the two closest beings of comparable power.

After Discord was defeated they were sealed in the garden along with Discord.

When discord escaped the trio were also freed. Do and Ray were furious and sought to find the Princesses and demand their powers back.

Screwball having had none of that went out to find her daddy and as such when the other two were returned to stone she was turned back as a seperate statue.

Do and Ray's statue was destroyed by Nocturne who flung Princess Luna through them along with a number of others in her failed assasination attempt. Screwball's statue is still some where in the garden.