The trio replace loose-forward Matt Smith, centre Alex Dunbar and lock Rob Harley, who have all been released back to their club Glasgow Warriors, having been added to the squad last week.
McDowall, 20, a former Scotland Under-20 international, signed his first professional contract with Glasgow Warriors last month.
Wing Lee Jones is a doubt with a knee injury and his fitness will be assessed this week, while prop Zander Fagerson and hooker Fraser Brown will continue their rehabilitation with the squad.
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Townsend has had to cast his net wide for the Six Nations after 19 players initially under consideration were unavailable for selection, including experienced flankers Hamish Watson and John Barclay, lock Richie Gray and both first-choice hookers Brown and George Turner.
(Reporting By Nick Said, editing by Pritha Sarkar)
