Many questions about a construction project

We are currently considering building with friends. The parents of our friends own some plots of land. A part was surveyed into 3 plots in 1999, and building was approved for 2 of the parcels. Now we want to build on it. Since one of the 2 parcels is only 400m² in size, the idea is to merge the 3 parcels into 2 parcels of equal size. How should we proceed now? What are the subsequent steps and what costs will we face?

Could you upload a sketch or a site plan?

That somehow sounds very expensive. And is that really necessary?

Maybe it would be enough to just subdivide one property again and leave it as is...

Parcels 1 and 2 were approved at the time, parcel 3 was not

If only parcel 1 and 2 are buildable, merging and redividing them won't help you either. The one with the northern parcel would have almost no building land. I would make an appointment in advance with the responsible building authority / district office and explain your request, maybe they will deviate from the previous decision?