If I have understood you right, I think you should try:
selections minbp_5 is greater than 3
This checks the minimum back price of the selection over the last 5 minutes. Replace 5 with whatever time you want to check. If you cannot live with the last 1, 2, 5 minutes then:
To accurately check the time since the race started it is (in minutes), I think you need to use:
(now_time - event_off)/0.000694
So if you set a variable, lets call it my_mins_gone = (now_time - event_off)/0.000694
then use the condition
selections minbp_my_mins_gone is greater than 3
I think that should work but have never tried joining a variable to minbp_ (does anyone else know this?)
Don't forget that just because the back price fell less than 3, that does not necessarily mean that it has been matched, so to check that you would need to create a new variable to record the minimum last traded price as (to my knowledge) there is no standard variable for minlt_{minutes} or maxlt_{minutes} (ALTHOUGH THIS WOULD BE A VERY USEFUL UPGRADE............ PLEASE)
good luck.