Have you done a web search to find out?

I know I need the Voodoo 3 drivers but what DirectX version do I need ?
And do I need any other updates and patches ?

The internet is full of suggestions but which ones are correct ?

The required DirectX version depends on the software you're intending to run on your system.
Version 6.x should be the current version upon release of the Avenger chip.
I don't think there's any software you'd want to run on this system that needs a newer DirectX version than 7.
If you need any other patches to run games they'd usually be included and installed together with the game. Many people didn't yet have internet access in 1999.

Since this thread is obviously related to your Win95 build (as you said here) I feel it would have been better to ask your questions in that thread. Maintaining a bit of discipline when creating new threads really helps keeping the forum clear and organised.

I have installed a Voodoo-3 on Win98se but never on Win95b. That's why I ask.
What Win95b updates would I need ?

Been 100 years since I ran 95b. These days I prefer 98lite with shell swap. Or 98 Micro if I want super lean.
If I remember correctly you need Dcom95, and Directx6. Not sure about the dcom95 part, but I know later versions of dx require it like dx8. Oh you might need MSI installer or windows installer updates, to install 3dfx tools.

3dfx drivers after 1999 will require DirectX 7.