In Catholicism, is an object a third-class relic if touched by a saint when they were alive?
The short answer is: No.
First of all let us see what relics are in the Catholic Church.
Classification and prohibitions in the Catholic Church
In Catholic theology, sacred relics must not be worshipped, because only God is worshipped and adored. Instead, the veneration given to them was "dulia". Saint Jerome declared, "We do not worship, we do not adore, for fear that we should bow down to the creature rather than to the Creator, but we venerate the relics of the martyrs in order the better to adore Him whose martyrs they are."
The Catholic church divides relics into three classes:
First-Class Relics: items directly associated with the events of Christ's life (manger, cross, etc.) or the physical remains of a saint (a bone, a hair, skull, a limb, etc.). Traditionally, a martyr's relics are often more prized than the relics of other saints. Parts of the saint that were significant to that saint's life are more prized relics. For instance, King St. Stephen of Hungary's right forearm is especially important because of his status as a ruler. A famous theologian's head may be his most important relic. (The head of St. Thomas Aquinas was removed by the monks at the Cistercian abbey at Fossanova where he died.) If a saint did a lot of traveling, then the bones of his feet may be prized. Catholic teaching prohibits relics to be divided up into small, unrecognizable parts if they are to be used in liturgy (i.e., as in an altar; see the rubrics listed in Rite of Dedication of a Church and an Altar).
Second-Class Relics: items that the saint owned or frequently used, for example, a crucifix, rosary, book, etc. Again, an item more important in the saint's life is thus a more important relic. Sometimes a second-class relic is a part of an item that the saint wore (a shirt, a glove, etc.) and is known as ex indumentis ("from the clothing").
Third-Class Relics: any object that is touched to a first- or second-class relic. Most third-class relics are small pieces of cloth, though in the first millennium oil was popular; the Monza ampullae contained oil collected from lamps burning before the major sites of Christ's life, and some reliquaries had holes for oil to be poured in and out again. Many people call the cloth touched to the bones of saints "ex brandea". But ex brandea strictly refers to pieces of clothing that were touched to the body or tombs of the apostles. It is a term that is used only for such; it is not a synonym for a third-class relic. - Relic (Wikipedia)
A third-class relic must be touched to either a first or second-class relics.
Just as some holy individuals may, with divine assistance, operate may miracles during their earthly lifes; these miracles are not to be counted as miracle used in the process of beatification or canonization. So too an object touched by someone to be canonized in the future is not a relic. A miracle for beatification or canonization must have occurred after the death of a particular Venerable or Servant of God.
The canonization of confessors or martyrs may be taken up as soon as two miracles are reported to have been worked at their intercession, after the pontifical permission of public veneration as described above. At this stage it is only required that the two miracles worked after the permission awarding a public cultus be discussed in three meetings of the congregation. The discussion proceeds in the ordinary way; if the miracles be confirmed another meeting (super tuto) is held. The pope then issues a Bull of Canonization in which he not only permits, but commands, the public cultus, or veneration, of the saint. - Beatification and Canonization
Although something touched by a future saint is not a relic, it still may carry with it a sense of notoriety. The objects were not truly touched by a canonized saint and as such are not relics. One is not canonized while still living! It is not uncommon to see plaques (especially in Europe) stating things like the following (some are now museums or placed in museums): St. "X" stayed here or laid here. There is some genuine notoriety with objects of this type, but they are not third-class relics.
- While in France, I can recall reading a plaque on a house in a small country village that simply stated "Here St. Jeanne d'Arc stayed". Nothing in the house was a relic, but it was so cool to visit a house that St. Jeanne d'Arc had stayed the night on her way to Orleans. I am sure some of the things in the house must have been touched by the Maid of Lorraine.
Third Class relics must be touched to either a first-class relic or second-class relics. It is that simple.