The three worlds or Tri-Loka in Hinduism are a generalized term for the three broad planes of existence/consciousness [further sub-divided in other planes], the Dev Loka, Bhu or Prithvi Loka and Patala or Narak Loka.
The Surya Mantra has to be chanted 1.25 lac times by the Sadhaka within 11 Sundays.
After that, he must write the Mantra on a Bhojpatra using Asht Aandh and then wear it on his right bicep. It the Sadhak does this, he will, definitely, without any doubt, win over all the three worlds.
Mantra: || ॐ घृणि: सूर्य आदित्यः ||
Other than what is specified by me above, the rest of the procedure of chanting the Mantra is mentioned in the post published earlier on the immense uses and benefits of this particular Surya Mantra. That post can be seen here - Benefits of Surya Mantra