Hi this reallu depends on what youre looking to do in later life. if your looking to go to college or univeristy, you are more than likely youll need a langauge so if taking 3 science will stop you from doing a language then its probably not in your best interests to take all three. but if you KNOW you wont need a langauge or if taking 3 science doesnt stop you from taking one. then you need to look at the course work for all the science classes and see if you think you can manage all of them especially if youre looking for HL in all of them. For me chemistry and physics is quite a handful but there are ALOT of links between them that allows you to cross over information. i dont currently take bio so i cant really judge but i do know that it has a lot of cross over with chem. the main issue with bio is that you have to be able to learn the notes. both chem and phy are more praticle/ logical if you understand the logic you should be fine to pass most test. a strong foundation in maths is also useful for chem and phy but isnt nesscessy so long as youre willing to put in the effort.
The physics course is also quite short, I'm in 6th Year and we're finished right now. It only requires JC maths
I actually have a friend taking all three of the sciences, she does get a fairly hefty workload, but becuase of the overlaps it's all manageable. It deponds on your school too, and how the subjects are being taught, but I will say: there are 24 mandatory experiments for each of the 3 subjects, so that would be a lot to learn.
I would recommend both chem and bio if persuing a career in most of the sciences, physics is more if you want to go at work at Cern, that sort of thing. The courses for all three are quite inclusive, obviously there's boring parts, but you'll enjoy lots too.
I don't think there is any course in Ireland that requires all three sciences, my reccomendation would be if that's what your interested in, go ahead andd choose all three, otherwise pick two and one other subject. Even just one science subject will take you a long way, so don't think you need all three. ;)
That part is really based on you if you find that you are really strong in the science then it would probably be similar but if you tend to be a bit weaker then it could be more. Most people wouldnt take 3 because hey do find it challenging to maintain high score in all 3 at HL,( a friend of mine is doing and is planning to take at least oone at OL so that he isnt under too much pressure but thats just him.) Where as business or history would give you a break from the complexity of the sciences (the science subject have a LOT of new material that isnt covered in JC espeically if you did the new JC course) but if youre willing to put in the effort then there is no need to worry.