1
Yes, diderot - there have been two teams since 1973 to have produced a team ERA under 3. Oakland in 1974 (2.95).
Yowza.
EDIT TO CORRECT: The 1973 start date accounts for the additional advent of the DH and I'm looking AL only since the league ERAs differ by 0.50 runs on average. The relevant question -> has any AL team since the DH and mound height increase ever done this. Answer: Just the one.