The Mystery of the Three Gongs

[HKPAX]

Any pilots out there to put my mind at rest as CX SLF?

SECOND gong after take off - cabin crew can get cracking on my gin & tonic.

THIRD gong - I can go take a leak.

But what does the FIRST gong signify??? (It sounds at just a handful of thousand feet while the a/c is too steeply pitched for anyone to be going walkabout in safety let alone comfort).

Thanks in anticipation!

HK Pax