It is not correct to state that you need to have held a PPL for 3 months prior to taking ATPL examinations
It is a JAA requirement to hold an ICAO PPL before you embark on a ATPL course
You can pre-read and pre-study with ATPL material
Some modular courses are 12 weeks long by distance learning others are 6-8 weeks long before the first stage of examinations may be taken
At the shortest then it is 6 weeks after holding an ICAO PPL if exam timings work out right for you
My advice to the topic starter
Study your PPL theoretical knowledge well
Only use ATPL material to cross reference to - but enjoy reading it