Biology and chemistry are looking good - they're the compulsory subjects so the third subject can be whatever you want: so history is fine. Don't know anything about the Welsh Baccalaureate though, sorry!
For dentistry:
1. Try to do your best academically so try to get AAA in your A Levels (and whatever the highest grade in the welsh bacc is)
2. Make sure you have dentistry work experience (minimum 2 weeks) as this is a must
3. Do some other extra-curriculars such as volunteering, sports, musics, reading around the subject, arts and crafts e.g. Knitting
4. Towards the end of year 12 (more like in the summer holidays) prepare for the ukcat/bmat
Good luck!