3 years fixed interest rate - Which banks do this?

So you’re planning with unlaid eggs. Who knows at what price the house will be sold in 1-2 years, if at all. I would finance your property in the usual way and negotiate a 10% special repayment. Then the debt can be paid off very quickly in case of doubt.

The usual way would mean 10 or 15 years and then almost 4% interest every year, that’s too much for me. I won’t do more than 5 years fixed interest, I prefer 3 years. I can still do the special repayment though. :)

You won't get significantly cheaper even for 3 and 5 years. How do you come to that conclusion?

Normally would mean 10 or 15 years and then almost 4% interest every year, that is too much for me. I will not do more than 5 years fixed interest, I prefer 3 years. I can still do the special repayment though. :)

4% interest every year on the remaining unpaid portion, minus the return you get when you invest the capital from the sale. Interest rates have risen on both sides. Calculate that in detail (there are good calculators online), and see if you really save something with the short fixed interest period. If you have to accept interest surcharges, probably not.
What speaks against the 5 years?

Normally would mean 10 or 15 years and then almost 4% interest every year, that's too much for me. I won’t do more than 5 years fixed interest, I prefer 3 years. I can still do the special repayment though. :)

Interhyp says 3.68% for 5 years, 3.81% for 10 years and 3.82% for 15 years. Or for 14 basis points or a 3% more expensive financing you buy independence from the other sale. Honestly, if you keep resisting that, it’s your own fault if something goes wrong. With numbers like these, I would even go for 15 years.

Then I'll take the 5 years. :)

My mother wants to sell her own house in 1 to 2 years and I will get almost all the money from the sale.

How about selling the house now and moving out in 2 years? Otherwise, of course, finance variably, or simply continue the current financing at the current rate. The advantage is that it becomes unattractive for the mother to change her mind after all ;)