It really depends. I dropped out of Year 12 in 2014 however returned in 2015 and ended up taking 3 subjects due to personal circumstances. However there was a worry that I wouldn't be able to progress to Year 13 due to my biology exam, luckily I got an E instead of U however if I hadn't received a pass, I wouldn't have been able to continue in Year 13. Although if that does happen you will probably have the option of picking up another A-Level and doing a year extra. Just keep it in mind that a fourth subject is there just in case one of your other subjects doesn't work out the way you wanted them to.