Ultimately, the best thing to do is put your favourite university as your firm and then a safe grade university that you still like as your insurance. So I think your best bet is to firm the ABB if that’s your favourite and then BBB/BBC is still a good insurance choice - plus, if they’re offering an unconditional if firm, then chances are they may still take you even if you miss their asking grades (and the ABB uni may also let you in if you miss by a grade or two).
If you firm an unconditional, you will be going to that university on results day - you can get out, but it can be a bit of a long winded process.
Also: this is NOT an ASAP situation. Your deadline is in May. You should NOT be picking your firm/insurance choices on Track until April at the earliest.
The way the system works is that you choose your firm choice ie the one you want to go to. Before results day universities get the results. If you meet the requirements your in. if you dont meet the requirements your firm universitity then decides whether they still wish to accept you or not. If they decide to accept you go there and your insurance falls. If you dont get accepted by your firm then it goes to your insurance university who will either accept you or place you in clearing so on results day you are either going to your firm, insurance or placed in clearing. There is no choice. With unconditional offers there is no insurance as you have a guaranteed place so you go to that university.
On results day you have 2 choices. If your grades are better than predicted you can apply through adjustment to a different university usually a higher graded university. If you are holding an unconditional offer you cannot use this. Alternatively you can reject your university and ask that university to put you into clearing. You then apply to another university who is accepting applicants. Universities dont rush taking students off and then the best places may have gone. This is basically how it works.