this may not be a very exact answer, because i don’t really have any teachings to quote for you.
when we use the word ‘liturgy’ we simply mean to say “rite” or “ritual”. liturgy is commonly used in an attempt to avoid the cultural connotations that have come to be associated with “ritual” etc. we use “liturgy” almost synonymously for “Mass” to signify that it is the central ritual of the Church.) thus when you hear that the Triduum is a single liturgy, what is meant is that these three days are one structured ritual that consists of more than one Mass: the Mass of the Lord’s Supper, the rites of Good Friday, including the Veneration of the Cross, the Easter Vigil, and the Mass of the Resurrection on Easter Sunday. just as each Mass makes the sacrifice of the Cross present to us again, the liturgy of the Triduum makes the event of salvation present to us also. it follows the events of Christ’s Passion, Death and Resurrection as they happened. the liturgy of Good Friday mirrors the friday on which the Lord died.
there is no reason, per se, why there is no Mass on Good Friday. it is that way now, because it is the ancient practice of the Church. in order to draw the faithful into the events which definitively secured our salvation, the Eucharist, which is always present in the tabernacle of the Church, is removed to the altar of repose, just as Jesus withdrew to pray in the garden and was later taken from the disciples to be put to death. the Lord, at the hands of others, gave up his life and withdrew from the world on Good Friday. so that we may become closer to sense of utter loss, no Mass is celebrated, so that we too can contemplate the loss of Christ in the tomb. a Mass would make Him present to us, but that is not an event of the original Sacrifice. Mass is celebrated on saturday night, since sundown is the beginning of Easter day itself. it also makes present the devotion of the disciples who kept vigil at the tomb after the sabbath ended, and who would later find the Lord risen at sunrise. because the Lord was known to be risen at sunrise, we don’t celebrate the Easter Mass until then.
all of the elements were put in place over time. that’s the basic scheme behind the Triduum. the development of the Easter Vigil as the main celebration of initiation also reflects the same kind of sentiment. the catechumens are prepared just in time to experience and participate in the risen presence of the the Lord.
keep in mind that Christ is not truly absent from us. a sufficient amount of the Eucharist is purposely reserved for the Communion rite on Good Friday, which is one of the rites of the day.