The 3 hour rule applies from the completion of the decontamination process which is the completion of the cycle. The print out from the machine will indicate when the cycle was completed regardless of how long it is before it is taken out of the machine, this is the time that is used.
If the scope is to go into the drying cabinet, then the time from the completion of the cycle and being put into the drying cabinet and the time taken for the lumens to dry within the cabinet should not exceed 3 hours.
For example, if the drying cabinet cycle time is 1 hour then the scope will need to go into the cabinet within two hours of completion of the decontamination cycle.
There should be a risk assessment of the stages of this process if there are any issues with regard to time taken at each stage.