I did dentistry, graduated 2 years ago so fairly recent idea of applicants.
If they want to go down the dentistry route I would say they need 100% need dental work experience plus another specialism of dentistry e.g. orthodontics, or maxfax. I had 4 weeks work experience and some of my friends had a lot more! Plus I would also say some volunteering over a period of time. Doesn't have to be in a care setting but if they don't have anything currently I would go down the care route. A lot of my interviews asked "why dentistry not medicine" so it can be useful to show you've experienced medicine and rejected it, so medicine based work experience is also useful
In terms of A levels, sometimes they will let people in with AAB, but only if they have space. So if they have 80 places, and give out say 150 offers, if all 80 places are taken by AAA students they won't, if 76 are taken and you have AAB they will. They have a quota which they aren't supposed to go over or under.
I did 4 A levels, and I would actually really recommend this. If someone is capable of medical/dental school they are capable of doing 4 A levels, it gets you in a much better mindset of working hard and I think with 3 I would have been bored. Plus I had wiggle room if I fucked up one of my extras.
I would also say a sport as well is good. I think most people on my course were also quite sporty. It gives you a break from work as well
I agree with the spreadsheet. Look at your strengths as an applicant and apply to universities that look for those qualities.