There is one version in Child where the third sister is mentioned after the first verse

There lived three sisters in a bouer
Edinbruch, Edinbruch
There lived three sisters in a bouer
Stirling for aye
There lived three sisters in a bouer
The youngest was the sweetest flower
Bonnie St. Johnston stands upon Tay

There cam a knight to see them a'
And on the youngest his love did fa

He brought the eldest ring and glove
But the youngest was his ain true-love

He brought the second sheath and knife
But the youngest was to be his wife

After that it continues in the usual manner. I do have a question, though. Who is St. Johnston?