According to Wikipedia:
Dan McCarney (ISU): since 2000, he lost three times to KU (2003, 2005, 2006). Fired in 2006.
Paul Rhoads (ISU): lost twice to KU (2009, 2014). Fired in 2015.
Matt Campbell (ISU): only lost this year to KU. Seat is getting warm.
Gary Patterson (TCU): only lost to KU in 2018. Fired 2021.
Guy Morriss (Baylor): lost twice to KU in 2003 and 2007, although he avoided playing the Jayhawks every year thanks to divisions (schedulemakers). Fired in 2007.
Realignment and divisional play messes up the stats a bit going back to 2000 (and after realignment KU was so historically bad I don't think they beat any team 3 times), but so far it looks like 3 is the max any coach in the Big 12 has been allowed in this millenium.