Third screen is not detected anymore after update

This machine MacBook Pro (16-inch, 2021) - Technical Specifications (CA) ?

Display Support

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

• Up to two external displays with up to 6K resolution at 60Hz at over a billion colors (M1 Pro) or
• Up to three external displays with up to 6K resolution and one external display with up to 4K resolution at 60Hz at over a billion colors (M1 Max)

JohnGeorge88 wrote:

Yes, I’m using a dock .. and it was working fine before

The point is: some docks just work, some others require third party software.

I am using a dock as well; it has HDMI and VGA ports, and works with my mac without installing anything else.

It continues to work in Ventura.

Some other docks necessitate the installation of third party software to be able to connect a display to your mac.

I am enquiring whether this may be your case. If it is, you'll need to check with DisplayLink for a version that works in Ventura (I don't know if they have one at the moment).

What are the displays and how are they connected?

Perhaps you used a dock that requires "DisplayLink" software to work with your mac?