The three wires are all live if its a 1 way switch and its in 2x L1 and 1x L2.
If this is a staircase switch (2-way) you will have 1x L1 1x L2 and 1x L3

How it works.. for a 1-way switch, think of it as letter T and we assume that one HOT live goes to ten lights.

the horizontal -------- is common HOT live feed from DB box ("common tree branch") to ALL TEN lights' switches - i.e. this common HOT is powering up more than one light and this is NOT subject to switch action (common HOT wire that runs before the switch to ALL the ten switches as HOT INPUT)
the vertical I is the ("one leaf only") circuit to that ONE light alone and is controlled by the switch to on/off that ONE light only.

So by assuming your EXISTING three wires are stuffed in a 2x L1 and 1x L2, the one with two wires is the horizontal section (common HOT)
the one with one wire is the individual ONE light circuit.
This can be verified by a test pen with the ONE light switch off.

This post has been edited by ceo684: Dec 9 2020, 09:35 PM

No issue but this switch (single pole) cannot be used for special purpose things like emergency exit/KELUAR sign (recommend to use double pole MCB so wont back flow current into the TNB grid as it has a transformer, some good design got diode to prevent this but some may backflow), or wet floor appliances (water heater need double pole), or for backup power transfer switch (for generator backup MUST use proper transfer switch else you may kill TNB people).

This post has been edited by ceo684: Dec 9 2020, 09:44 PM

Take pic and label the wires one by one according to where they came from (follow OLD switch labelling). If all else fail at least can revert rather than trying to guess what goes where in 3x3 = 9 possible combinations