I’ve previously covered how Katey Sagal had quite an extensive music career in the 1970s and 80s before her acting career took off, often performing backup vocals for famous artists. At one point, she was a member of The Harlettes, a trio of backup singers who performed during Bette Midler’s musical numbers. She’s the singer in the middle.
UPDATE: According to @ButterWanker on Twitter, “That’s NOT Katey Sagal. It’s Ula Hedwig. That Bette Midler HBO special was in 1976, Katey wasn’t a harlette until 1978.”