If you are polite and use manners, and explain to them that you sort of want to really succeed by giving yourself time to prepare well, then I do not think they will ever see you as a "problem case".
If you are going to put the application on hold, then you can probably arrange with them so that when you are fully ready, you can contact them to book another date for Phase 3.
Always ask politely though, and try to show them that the main reasons for doing so is that you want to get on this scheme, no matter what.
NB: In my opinion, giving yourself time will also enable you to be more calm during the interview - which is also very important I guess.