Anyone who "guarantees" a PPL in as short a time as three weeks should be regarded with some suspicion. I knew of a mob that did that, and the only way they could jam the students through was to do some very dodgy paperwork fiddling.
The quickest one we have done was three weeks and two days, and he had all the theory squared away before he started.
On the other hand we had one chap came to us who had done a three week PPL in the states, and the only way he had got through was by big chunks of the syllabus being missed out, including the IF. And because of the lack of any kind of consolidation time for things to "sink in" his confidence in his flying wasn't the best, because he knew he'd forgotten much of what little he had done.
IMHO it's better to take your time and allow at least five weeks for a full time course, you need to have some rest.