From Halopedia, the Halo wiki
Three Gates is the sixteenth track (Disc 2 Track 1) included in Halo 3: Original Soundtrack, and plays in the Halo 3 campaign level The Covenant. It is composed of Three Gates (0:00-0:47), Ascent (0:47-2:02), and Leonidas Returns (2:02-4:34).
Three Gates (0:47) begins with a low strings note, which is followed by the piano playing the five-note E Dorian melody (from the later track Finish the Fight) and a variation of the phrase to answer the melody as percussion joins in. The piano plays yet another variation of the melody, with cymbals leading into heavy percussion and low piano notes from Unyielding (in Halo 2: Original Soundtrack). The piano stops abruptly, with the heavy percussion taking the lead until it slowly fades out. The piece plays in the opening cutscene, and leads into the opening gameplay encounter. Whereas the soundtrack version ends with a fade, the in-game version with a percussion climax.
Ascent (1:15) fades in with soft percussion and bass. Strings enter to play an accompaniment in the background. The strings melody slowly ascend into high notes until the percussion takes over and then stops. The bass-only version of the piece plays when John-117 takes the elevator to the top of the first barrier tower, and the full version of the piece plays when the UNSC forces return to the beach. The bass part was played briefly later in the level, after the battle with two Type-47B Scarabs, when it was revealed that Johnson had been captured and taken to the Installation 00 Citadel.
Leonidas Returns (2:32) is a full rearrangement of the piece Leonidas from the track Delta Halo Suite (in Halo 2: Original Soundtrack), featuring the addition of brass, more prominent low strings, and a wider range of choir. This piece plays during the chapter "If You Want It Done Right...", when John-117 pilots an AV-14 Hornet to reach the third barrier tower.