UNSC Battle Group Sierra-3

{{Infobox Military Unit }}
 * unit_name=UNSC Battle Group Sierra-3
 * image=
 * dates=Early 2543
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 * allegiance=UNSC
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 * size=3 ships
 * 1 Cruisers
 * {UNSCShip|Io}}
 * 2 Destroyers
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 * battles=Battle of 18 Scorpii
 * decorations=
 * battle_honors=
 * commanders=Unknown UNSC Captain
 * battle_honors=
 * commanders=Unknown UNSC Captain

UNSC Battle Group Sierra-3 was a battle group that fought in the Battle of 18 Scorpii that spurred Vice Admiral Preston Cole to seek out the Covenant at Psi Seprentis leading up to the Admiral's last stand.