Updated
List of characters where third hit of 11f1 will always land AFTER doing a F3/21. For some characters listed, the third hit will whiff after blocked ex fb. The number in brackets indicate which part of the string gets blocked. All the characters in the "midscreen" list will also work in the corner. Also, for some characters you must follow up 11f1 quickly.
Midscreen: Scorpion (23), Kung lao (23), liu kang (13), ermac (13), nightwolf (23, 90% of the time), jade (13), stryker (13), jax (3), raiden (13), shang tsung (13), sheeva (13), skarlet (23), kenshi (13), rain (13), Freddy (23)
Corner: sub zero (23), scorpion (13, 23), sonya (23), freddy (13)
Gonna do another list for characters guaranteed to be hit by 11f1 after ex fb.