1. Tantum Ergo:
In fact, when we sing this anthem, we are just singing the last two verses of a longer hymn, the Pange Lingua Gloriosi, written by St. Thomas Aquinas who, even though he is way better known as a theologian than as a hymnographer, wrote probably the most beautiful Eucharistic hymns, including this one.
Traditionally, the full Pange Lingua is sung on Holy Thursday and on the Solemnity of Corpus Christi. Today, the Tantum Ergo is practically a pillar of the Roman liturgy, specifically during Eucharistic Adoration.
2. O, Salutaris Ostia:
See? Another hymn written by St. Thomas Aquinas! This hymn, just like the Tantum Ergo, is actually composed of only the last two verses of one of the hymns sung on the feast of Corpus Christi, the “Verbum Supernum Prodiens.”
Along with the “Pange Lingua,” this hymn was written at the request of Pope Urban IV, who instituted the feast of Corpus Christi in 1264. Today, the “O Salutaris” is more often heard, also, during Eucharistic Adoration
3. Ave, Verum Corpus
Nope. Not by Aquinas. Not this one. This beautiful 14th-century Eucharistic hymn has often been attributed to Pope Innocent (nobody knows whether that’s Innocent II, III or IV, by the way), but no historian knows for sure who is responsible for it. That’s precisely the reason why we would rather share Mozart’s version (which is, needless to say, breathtaking!).
In the days of the pre-Tridentine liturgy (that is, prior to the liturgical reform of the Council of Trent, in the 16th century) this hymn would be sung during the elevation of the Host at Mass. Nowadays, the “Ave Verum” is associated more specifically with Eucharistic liturgical feasts and, in some countries, even with Christmas time.