Erin McQuade, 18, and her grandparents Lorraine, 69, and Jack Sweeney, 68, died when they were hit by the vehicle on Monday December 22.
Six people were killed and 10 more injured after the lorry lost control in the city's George Square.
A requiem mass for the three victims was held at St Patrick's RC Church in Dumbarton, West Dunbartonshire.
The private service was led by Archbishop Philip Tartaglia, the Archbishop of Glasgow.
He previously told a memorial service that he cried with the mother of Miss McQuade who had watched her teenage daughter and both parents die almost right in front of her.
He told St Andrew's Cathedral: ''On the evening of the tragedy, I was privileged to be permitted to spend some time with one of the families who had been cruelly devastated by the incident.
''I was able to witness and share the grief and sadness of a mother and of a father for their daughter, and of two daughters for their mother and father.
''The distressed woman to whom I was speaking had been at the incident, she had seen her daughter and her own parents killed almost right in front of her. Can you imagine the horror? Can you imagine her sadness?
''I tried to console them and comfort them. We spoke and we cried and we were silent before the abyss of their loss and the random meaninglessness of what had happened.''
Today he told the congregation that the accident had put "tragedy on tragedy and sadness on sadness" for the family.
He will say: "Jack and Lorraine's daughters, Jacqueline and Yvonne are also Erin's heart-broken mum and sorrowing aunt. This is a family devastated by the tragic deaths all at once of three much-loved members.
"They were struck down in front of Jacqueline's eyes. A festive and happy Christmas shopping excursion to Glasgow had become the worst of nightmares. What happened was random, cruel and meaningless."
The service will hear how the Sweeneys had been "very happily married" for over 45 years and that their granddaughter was a "light-.hearted, caring, family-loving girl".
He'll say her parents Jacqueline and Matt will "always mourn for her" and will "always feel the searing pain of her loss".
They "died as they lived - together" and "it is fitting that they should share the same funeral Mass. They will be buried in the same grave," he will add.
Primary teacher Stephenie Tait, 29, and Jacqueline Morton, 51, both from Glasgow, and Gillian Ewing, 52, from Edinburgh, were also killed in the accident.
Three people are still in hospital.