Eugene Lewis had described the allegations as "absolute rubbish . . . they never happened" and showed no emotion as a jury found him guilty of 11 counts of indecently assaulting the three sisters.
During Lewis's trial it was also alleged he later raped one of his victims after she visited him for counselling over an affair she had with a married policeman.
Omagh Crown Court was also told that Lewis, who is based in Dublin, had committed serious sex crimes in the Republic.
One victim told the court he raped her twice in the White Fathers' house in Dublin. The court also heard a claim by a fourth sister that she was sexually abused at the Order's former home in Blacklion, on the Fermanagh-Cavan border.
However, these allegations were not the subject of charges as they had been allegedly committed in the Republic and outside the court's jurisdiction.
Gardai could not confirm whether they had investigated, or received complaints about Lewis.
A garda spokesman said last night: "Unfortunately we are not in a position to comment on a matter that has been before a court, or on named individuals."
The serious sex attacks were outlined by prosecutor Ken McMahon who said the priest had wormed his way into the family through another, innocent cleric.
Mr McMahon told the jury that although welcomed into the family's home in the North at anytime, Lewis often chose to call at bedtime or on Saturday when the girls were bathing.
One of Lewis's victims told the court he abused her while telling her and her sisters bedtime stories.
However, when she was aged 21, the priest raped her at the Templelogue base of the White Fathers after her parents sent her to him when they discovered she had an affair with a married policeman.
The court heard she was left traumatised, but that he raped her again the following night despite her begging him to stop.
She later complained to her mother about the abuse, although she said nothing of the rapes. But it was not until two years ago that she finally went to the authorities in the North and South.
It took the jury of six men and six women four-and-a-half hours over two days to unanimously convict the former provincial superior of the Society of Missionaries of Africa, better known as the White Fathers, who was based at Cypress Grove House, Templelogue, Dublin.
The charges related to differing dates between August 1963 and September 1973.
The priest, who told police and the jury that he "never touched those children", will be sentenced next month after pre-sentence reports are completed.
Joe McVeigh, defending, said that while his client respected the decision of the jury, "he is very disappointed and he wants to stress that he remains adamant in his denials".
Mr McVeigh said that Lewis's defence team are now to "consider the position and give some thought to our grounds of appeal".SIC: II