Luplow has been on the injured list since May 28 with a sprained left ankle. He initially got hurt while working out in Arizona before camp opened. The 27-year-old was batting .173 with seven homers and 20 RBI in 36 games.
Johnson spent most of the season at Triple-A Columbus, going 1-2 with six saves and a 3.32 ERA in 21 relief appearances.
The right-handed Armstrong has spent parts of seven seasons in the majors. Armstrong has an 8.55 ERA in 20 appearances with the Orioles this season and was with Triple-A Norfolk at the time of the deal.
Tampa Bay entered Friday trailing first-place Boston by 1 1/2 games in the AL East and leading the AL wild-card race.