New on truenas and raidz pls help

Hi
Iam total new on TrueNas
I used a WD cloud pr4100 at the moment.

I would built my Nas with this
Supermicro X8DTI-F,
2x Xeon E5530 (Quadcore mit HT 2,4ghz Turbo 2,66ghz 8mb Cache ), Kühler + Lüfter,
128GB RAM

For start i have 3 x 20tb Disks for a Pool
Can i make a raidz1 and add later a 4th disk or nedd it to start all Disks ?

Thx for help

You will be able to do that with Truenas Electric Eel. Its currently in BETA. Release is probably sometime in October. Also while not new to ZFS, this feature will be new to Truenas, so be aware of bugs.

Can i start now with this Version and Update to the new or must i reinstall the new one ?
1ssd os
4x12tb raidz1
Next month Update
3x20tb raidz1
Copy files
Complete 3x20tb to 4x20tb raidz1
2x20tb as mirror

You can update later.