I'm not sure, but I think that the Christmas Mass at Dawn may be taken as the first Mass of Christmas Day (after the Midnight Mass, of course), so if the first Mass is at 8am, it would qualify. This is how we did it at St. Thomas Aquinas in Charlottesville way back when I was the choir director for the 8am Mass. If there were also a Mass at, say 6:30am, then the 8am Mass would not be the Mass at Dawn; i.e., there can only be one Mass at Dawn.
GIRM no 34: On the day of the Nativity of the Lord, following ancient Roman tradition, Mass may be celebrated three times, that is, in the night, at dawn, and during the day. [not much there]
Roman Missal, pp. 172: On the Nativity of the Lord all Priests may celebrate or concelebrate three Masses, provided the Masses are celebrated at their proper times.
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