Getting into NDT
A Level 2 rating requires specific documented work/study as a NDT tech for each specialty (Dye, Mag, Eddy, etc) for extended length of time plus testing requirements. For example, 1200 mhrs dye penetrant, 1500 mhrs eddy current. Most Level 2 A&Ps I know spent years working in NDT either non-aviation or in a Part 145 shop and obtained their A&P after their Level 2.
As for a Level 3, outside of established MROs or Repair Stations, independent Level 3 techs are few and far between. Unless you spent your career in NDT a Level 3 rating is not attainable--with or without an A&P. The only Level 3 A&P I know obtained his A&P concurrently with his NDT rating, but it took 25 years to obtain the necessary documented experience to reach Level 3 and qualify to take his A&P tests.
Hope that helps.
W1