1. 4 mm² for each of the 3 wires
2. C25 or C20
3. Direct wiring with disconnect switch, commando plug or round pin 15A socket + plug

This is not allowed by code in Malaysia. Only wiring larger than 16 mm² may use a Earth wire that is smaller than the power conductors.
(Disclaimer : this is off the top of my head, getting ready for something so reluctant to look for the actual size of wire but it is something like that. Maybe it's larger than 10 mm²?)

You want to save money by using 2.5 mm² Earth wire then that's on you.

Actually can use 2.5 mm² wires, as 3000 W will use under 14 Amp. 2.5 mm² wires are rated at 20 A so should be quite safe. Just ensure the wire is a branded that follows the proper specifications and the size is actually 2.5 mm². Make sure the MCB is C20 if using this size wire. I would use a maximum oven if using 2.5 mm² wires up to 4000 W.