Hello,
If you have not registered for BCECE counseling then you can't appear for BCECE spot round. You must have taken part in the first two rounds.
Also Spot round of counselling will only be held if it will be required. After the first and second round of counselling, if there will be vacant seats then the further round of counselling or spot counselling will be held for the candidates. Spot counselling will be the on the spot in which the left out candidates can come and get admission after fulfilling all the requirements. The board (BCECE)will conduct the third round or spot counselling in the same procedure if required at that time.
Hope it helps