More disadvantages than advantages usually.
There are just a few schools like Kengsingtonia's that insist on separate sciences to do A levels, so do check this, but normal schools (ie not independent or highly selectives will virtually all be OK with double). If a likely sixth form does have this stipulation then triple is a must for anyone who just might want to study a science at A level.
disadvantages are it either reduces other option choices at GCSE giving a less diverse selection or increases the workload if done as an extra on top of everything else (this last isn't a problem for an academic child who enjoys work).
It's not a fantastic syllabus - the extra stuff is covered very superficially (as with the rest of GCSE science - ie: more dross (can you tell I'm not a great fan of science GCSEs?). It really can be made up very quickly indeed if not done before A level.
Advantages are an extra high grade GCSE for a scientific child, and it's a good easy option for someone who is that way inclined but who would find another essay based subject or a language or whatever much tougher.
Triple does increasingly carry a sort of status with it as often only the more able are 'allowed' to do it, so looks good from that respect. Once you have science A levels no-one will care if you have it or not and arguably something a bit tough looking like another language might look better at GCSE, but if you do arts A levels then triple science at GCSE looks good - like you were a bright kid who could have done either but chose arts. Though to be perfectly honest, hardly anyone is going to analyse your choices in that much detail.