This is an old article from 2011. It is still correct, but I suggest you take a look at Should I Become A Driving Instructor for more information.
Someone found the blog on the search term “what happens if you fail your 3rd adi test”?
You have two years to complete all the training from the date when you pass your Theory Test (Part 1). Obviously, you can do as much training as you like before this, as long as you’re not giving lessons for money – and you can have as many goes as you like at Part 1.
Part 2 is the driving skills bit. You can have a maximum of three tries at this within the two year period mentioned. If you fail a third time, you have to wait until the two years of your Part 1 is finished before starting all over again – with Part 1.
Part 3 is the instructional ability bit. Again, you have a maximum of three tries within the two year period and – irrespective of how many tries at Part 2 you had – if you fail a third time you have to wait until the two years from when you passed Part 1 is up. Then you have to start with Part 1 all over again.
There is nothing which says you can’t do Part 3 training while you’re studying for Parts 1 and 2 (and ditto with Part 2). There is nothing which says you can’t teach people for practice purposes at any stage throughout this (every parent in the country would be committing a crime otherwise). The only thing you cannot do is take money for it – you need a PDI (trainee) licence to do that.