If you fail a year, what happens depends on your university. Usually you'll get one chance to resit (in August, probably) but your marks will be capped at 40% (i.e. you can only get a 3rd). If you fail that, then you have to retake the year (or drop out).
You have to consider whether you would be better off not giving in and pushing on till the end, so you do as well as you possibly can, or whether it would be best to drop out now and start something else. Presumably you're already 2/3 of the way through your degree and if you dropped out, would you regret it?