sorry I noticed immediately after: P but there are still some things that I have not understood:
1 can I catch the three legendary birds and Mew?
2 can the Pokemon League be challenged?
3 Is HM Forza available somewhere?
1. Yes, but you need to wait until after you beat the main game. You need Strength for Moltres and Articuno. I don't know where Mew is.
2. No.
3. Yes, Miss Hoenn gives it you. Follow these steps.
Miss Hoenn gives it to you after you get her out of the prostitution life.
1. The Underground Paths are brothels.
2. At the end of the Route 5-6 one, a Cooltrainer is with a redhead, that redhead is Miss Hoenn.
3. The pimp on the Route 7-8 Underground Path wants Miss Hoenn.
4. Go get Miss Hoenn, this will involve battling that Cooltrainer.
5. She wants a new career, she'll make that clear.
6. Nurse Joy in Pewter City suspects her Viridian cop boyfriend is cheating on her.
7. Go to Viridian, confront the cop, battle him, and you'll learn that he was just watching after a Jynx.
8. Go to Nurse Joy in Pewter, tell her that info, she'll reward you with a Red Flute. She'll then say she wants an extra for her Pokemon Center job.
9. Go back to the Route 7-8 Underground Path to get Miss Hoenn. You'll have to battle the pimp.
10. Return to Pewter City and you'll see Miss Hoenn with Nurse Joy, she'll give you HM04.