Taken from wikipedia though there are numerous website that mention the stages of endo and some have illustrations too:
Stage I (Minimal)
Findings restricted to only superficial lesions and possibly a few filmy adhesions
Stage II (Mild)
In addition, some deep lesions are present in the cul-de-sac
Stage III (Moderate)
As above, plus presence of endometriomas (over 2cm) on the ovary and more adhesions.
Stage IV (Severe)
As above, plus large endometriomas, extensive adhesions.
You sound like your somewhere between a stage 3 and 4.
Stage 4 is the depth of the endo lesions and usually indicates that they have grown deep in to the walls of other organs, and the glued up adhesions are extensive sticking your organs to each other.
As for your post op symptoms, they sound like you possibly had some diathermy (laser treatment) done on existing endo spots. This burns you inside, and the side effects are as if you have raging sunburn inside which like real sunburn does calm down eventually and turn very itchy inside too. I had extensive diathermy done and the itchy insides lasted longer than the sunburn feeling inside but it took weeks to get cleared up. Quite normal.
You will get pains and twinges and tugs and groans down below until your organs have found the right place to settles down again, seeing as they had to make way for the 10cm grapefruit endometrioma, now there is rattling around room and your freed up organs will lead you a merry dance of pains till they find a comfy spot to settle down in to again.
Be careful of any signs of infection which is hardened tummy swelling, red sore skin at the wound sites and feeling sick and feverish. In which case see the GP or call your gynae ward and speak to them. At 3 weeks you are probably over the worst of it by now and it should begin to ease off gradually.
You've had a humungous op and been poked prodded and stabbed and it does take quite some time to return to pain free normal. 3 weeks is nothing in recovery terms, you'll get there eventually, be kind to yourself and don't rush to recovery or overdo it and have a set back.