This question gives good opportunity to express both how the word “saint” can be used in multiple ways, and how the process of declaring persons as saints has developed and changed through the centuries.
While today there is a formal process by which persons who have died are declared saints called “canonization,” such a process has not always existed in a formal way. In fact, those men and women whose lives are most directly known to us through the Sacred Scriptures – that is, the Apostles, the Virgin Mary, Joseph, Mary Magdalene, etc. – were never subjected to a process of being canonized. Rather, it is by way of what is both revealed in the pages of the Bible referring to their lives (and deaths – in such persons as John the Baptist and Stephen, for example,) and in how they were traditionally venerated and celebrated by the early Church down to our own day that we have the “cause” for their sainthood, if you would. For example, to speak of one of them, St. Peter is known (both in secular history, and in the Church’s own early writings) to have died by way of “upside-down crucifixion” in Rome at the wishes of the emperor Nero around 64 A.D. To this day, one can see how Peter was celebrated by the early Church and throughout the centuries, as his gravesite (beneath the Papal Altar in St. Peter’s Basilica) attests to his being venerated in the early Church. Such veneration continued throughout the early Church, culminating with the first Christian emperor of Rome, Constantine, building the first and original St. Peter’s Basilica on this very spot in the early 4th Century. Was Peter’s sainthood subjected to a tribunal to decide on whether or not he was really a saint? Not to the extent of today’s process. However, because of the life (and death) that he lived, and the immediate upholding of the sanctity of his life by the faithful of early Church, he has since the earliest times been considered a saint.
I bring up Peter as an example of “early Church sainthood” mindful that when we use the term saint, we usually do intend to refer to human beings. However, the Archangels Gabriel, Raphael, and Michael share something in common with the biblical “human” saints: they lived and faithfully carried out to the end the very purpose that God had given to them. In this way, that these angels might be called saints is of no difficulty – as these three most certainly were faithful to God and his saving plan according to the particular purpose that each has been given. Furthermore, like all the humans whom we believe to be saints, these (and all faithful angels) dwell in God’s perfect presence in heaven. Finally, (and here is where Peter is once again important,) their memory for fidelity and holiness has been venerated by the Church throughout its generations – which, in the case of the archangels, is known to us mainly through their stories in the Scriptures. Thus, just as Peter was not subject to a “canonization process,” nor were these angels. Rather, their “sainthood” is attested to by tradition, rooted in the upholding of their sanctity by all generations of the Church.
On the flip side, now that all who are declared saints are subject to a process of investigation of their lives and death, I can safely assert that you will not hear about any angels having “causes for canonization” (as the opening and carrying out of such investigation is called) – as it is only human beings that can be subjected to such investigation – and therefore only humans can be declared saints in light of the process that now is in place.