You're probably too late, the ordering date to get a model 3 for this year has (almost) passed. They're saying that the order has to be confirmed before today 22:00 (oct 6), otherwise Tesla cannot guarantee the 2019 delivery date.

https://www.mistergreen.nl/tesla-model-3-lease

I'm ignorant of Dutch law, so I may be off base, but I find taxes usually apply as of the date of the contract, not the date of the delivery. So, if you have a firm order, you are probably fine.

I'm pretty sure that here in The Netherlands it applies to the date the vehicle receives a license plate.

The only think to remark is that ?two? years ago the same situation occurred with the Mitsubishi Outlander PHEV, thousands were ordered and the end of the year was approaching. A couple of people worked around the clock to register them all before the 1st of January to ensure the tax benefits (At least that is what I heard at that time).