From the Introduction to the Lectionary for Mass:
“79. In Masses assigned three readings, the three are to be used. If, however, for pastoral reasons the conference of bishops has permitted use of only two readings,
[footnote 105: 1975 General Instruction of the Roman Missal, no. 318; Congregation for the Sacraments and Divine Worship, instruction Inaestimabile Donum, 3 April 1980, no. 1; AAS 72 (1980) 333-334].
the choice between the two first readings is to be made is such as way as to safeguard the Church’s intent to instruct the faithful more completely in the mystery of salvation.”
The next question is which country’s Conferences of Bishops have permitted the use of only two readings. I don’t know the answer to that.
The 1973 Directory for Masses with Children also discusses fewer readings, including: “43. If all the readings assigned to the day seem to be unsuited to the capacity of the children, it is permissible to choose readings or a reading either from the Lectionary for Mass or directly from the Bible, but taking into account the liturgical seasons. …”.
