any suggestions as to how to say "if x &y&z are all low for more than x ms...",
You need to periodically check all values. If all are low, set the time that they went low (or, more accurately, the time they were detected to be low). If any goes high later, clear the time.
Periodically, see if the difference between now and then is greater than the interval of interest, when then is not 0. If so, stop.