So I’m speculating that 3 is L, now check where 1 and 2 go.
Your installation may be of this type, although there are other possibilities.
You must draw a detailed diagram of this installation. What is connected to what and what the individual wires are responsible for and where each wire has its beginning and end.
First determine where the main AC L and N are. Then determine how the lamp(s) and non-smart switches are connected.
Without this, it may turn out that the selected Sonoff mounting point is wrong, in other words, it must be at the beginning and only from there the rest of the pasta will be added to the lamps and non-smart switches.
Here you have an example of a simple solution, one sonoff and one non-smart switch, one termination point (lighting), main (AC IN)…
Determine where your diagram starts, probably the box in the wall. There must be the first termination point of L and N somewhere…
In this model, the N wire coming from the main goes directly to the final termination point in the lamp.
The L wire from the main enters the sonoff at L-In. L-Out further supplies L with voltage to the lamp.
S1/S2 are for a regular switch.
You, however, need to determine the connection diagram for your installation, it’s hard for me to speculate… Because it is the most important thing to do it correctly. Either it won’t work or you’ll electrocute someone.
I can only guess… You say you don’t have N, N must be in the installation, but you may not have N in the switch boxes, which happens. But N must be at the end of the installation per point.
If you don’t actually have N in the switch boxes, you don’t have to think about it. Focus on L and connections to switches.
If you want, look at the socket on the wall or under the lamp mounting on the ceiling, there should be at least two wires, i.e. L N, correct installations should also have a third PE wire (grounding), but… Electrical installations are a nightmare, depending on the country and year of construction and who did it, situations vary dramatically and you can never assume that something is in accordance with the regulations.
For this reason, sometimes it is better to hire a licensed electrician.