DO NOT DO THIS! You'll blow up your house.
Here's the problem. It's subtle.
By disconnecting neutral from utility, but not disconnecting the other two phases, you create a "Lost Neutral" situation between those phases.
There will be 400 volts between L2 and L3. However, there will be nothing keeping either one of them at 230V. All the loads on L2 (together) will be in series with all the loads on L3 (together). In other words, L2 will act like a split-phase North American system, but with 400V at the ends, and absolutely nothing holding neutral in the middle. And then, it will behave like a "Lost Neutral" scenario.
So the two phases will add up to 400V, but might be 100/300V or 50/350V depending on the loads on the two phases. Obviously that is more than they're rated for, so it will burn stuff out or set it on fire.
Mind you, all this is happening on L2/L3 after utility power returns. Up until that point, everything will seem like it is working.
The simplest answer is "Don't switch neutral".
That keeps neutral attached to the utility, so phase L2 and L3 will stay at their proper 230V-to-neutral voltage.
You would connect it to the generator also, but you'll need to comply with local Codes as far as whether the generator has its neutral and earth jumpered together.
If those codes require. you to switch neutral, then by golly, you'll need a 3-phase switch (4 pole). Leave earth connected. Connect the generator on 1 pole and neutral. The switch interrupts the other 2 poles just to keep the above problem from happening.
"I can just use 2 switches right?" No. They must throw together as one action.
That switch isn't nearly big enough
The problem isn't how much power will flow through it on generator.
The problem is how much will flow while on utility. It needs to be rated for full utility current - whatever your circuit breaker is.