What actors and actresses appeared in two or more consecutive sitcom flops before landing a role in a successful show?

Debra Messing before Will & Grace starred in Ned & Stacey and Prey.

Ok, technically Prey was not a sitcom but nonetheless, two flops before a sitcom success.

Most of the Friends cast would qualify.

Jennifer Aniston
Molloy (1990)
Ferris Bueller (1990-1991)
The Edge (1992-1993) (Sketch Comedy)
Muddling Through (1994)
Friends (1994-2004)

Matthew Perry
Second Chance / Boys Will Be Boys (1987-1988)
Sydney (1990)
Home Free (1993)
Friends (1994-2004)

Matt LeBlanc
TV 101 (1988-1989) (Drama)
Top of the Heap (1991)
Vinnie & Bobby (1992)
Friends (1994-2004)

Sofia Vergara
Hot Properties (2005)
The Knights of Prosperity (2007)
Modern Family (2009-2020)

Poor Things! Actors/Actresses Who Had A Couple Or More Failed Series (https://www.sitcomsonline.com/boards/showthread.php?t=337884)

McLean Stevenson had a number of flops since he left MASH. The McLean Stevenson Show, In the Beginning, Hello Larry and Condo.

Patricia Heaton had a string of failed sitcoms before hitting the big time with Everybody Loves Raymond:

Room For Two (1992-1993)
Someone Like Me (1994)
Women of the House (1995)

Ty Burrell had two failed sitcoms before Modern Family:

Out of Practice (2005-2006)
Back To You (2007-2008)

Robin Williams was a star, right out of the gate! But that's unusual.

Steve Carell

Over the Top (1997)
Watching Ellie (2002-2003)
Come to Papa (2004)
The Office (2005-2011; 2013)

If we ignore that whole One Day At A Time thing, Valerie Bertinelli had two sitcom flops, Sydney and Cafe Americain, before finding success with Hot In Cleveland.