Straight from the Regularity Authority to you -
"In the en-route configuration existing at the end of the level flight acceleration
manoeuvre, an aeroplane must be able to achieve a gross gradient of climb of
at least:
(a) for a twin-engined aeroplane — 1.2%; or
(b) for a 3-engined aeroplane — 1.4%; or
(c) for a 4-engined aeroplane — 1.5%."
That describes the 4th segment. In the 3rd segment the climb required is zero, but the aircraft must have the CAPABILITY of meeting a 1.2% climb anywhere in the 3rd segment (It helps in a number of OEISID creation problems).
Obviously then, the A320 in your example must meet a minimum climb gradient of 1.2% Gross (0.4% Net), it will, of course, do better than this at lower weights.
That's from the Australian Regulations (CAO 20.7.1B), these fairly much 'parrot' the FAR and JAR requirements.
Regards,
Old Smokey