Hyrule Castle Town:
After collecting all three Spiritual Stones, return to Hyrule Castle Town. You'll witness a dramatic cutscene involving Ganondorf and Zelda. Follow Impa to Kakariko Village. Then, head to the Temple of Time.
The Master Sword:
Inside the Temple of Time, place the three Spiritual Stones on the altar. Play the Song of Time (learned from Zelda's cutscene) on your Ocarina. The Door of Time will open, revealing the Master Sword. Pull the sword from its pedestal to become the Hero of Time, seven years in the future.