Where is the 36 month ATPL theory exam validity before CPL/IR test listed and described?
I don't know much about EASA regulations, but this seems to be a reference to Part FCL.025(c)(1):


  (1) The successful completion of the theoretical knowledge
  examinations will be valid:
  [...]
  (ii) for the issue of a commercial
  pilot licence, instrument rating (IR) or en route instrument rating
  (EIR), for a period of 36 months;
  (iii) the periods in (i) and (ii)
  shall be counted from the day when the pilot successfully completes
  the theoretical knowledge examination, in accordance with (b)(2).


Having said that, (c)(2) gives a much longer validity period for the ATPL exams:


  (2) The completion of the airline transport pilot licence (ATPL)
  theoretical knowledge examinations will remain valid for the issue of
  an ATPL for a period of 7 years from the last validity date of:
  (i) an
  IR entered in the licence;
  or (ii) in the case of helicopters, a
  helicopter’s type rating entered in that licence.


My understanding of that is:


Your IR theory exam is valid for 36 months from the date of the exam
Your ATPL exams are valid for 7 years from the date you receive your IR (although the wording is ambiguous, in my opinion)


It's possible that your provider is combining those two points somehow. The easy thing to do here is simply ask them to explain their statement in more detail and include a reference to the regulations.