Hello, lidia goluch.
Welcome to Microsoft Community!
I understand the discomfort this situation has caused you.
It's easy to see that your monitor is working fine, and this has only happened with the new Surface Go 3.
We can try to adjust the monitor settings.
Check monitor settings: Make sure your Samsung monitor's input source is set correctly. For example, if you are using an HDMI connection, make sure the monitor is set to HDMI input.
Change port version: Some monitors allow you to switch between different port versions (e.g. DisplayPort 1.1 and 1.2). Try changing the port version in the Setup menu of the monitor.
Additionally, you can also try to manually update the Surface Go 3 drivers and firmware to troubleshoot potential driver issues:
Download Surface Go 3 Drivers and Firmware from Official Microsoft Download Center
In order to determine the issue further, we do need to consider the compatibility of the cable with Surface devices, which tend to have higher compatibility requirements for external accessories, and there has been no shortage of feedback from users in the past about such issues.
We would recommend purchasing a Surface accessory cable specifically designed by Microsoft for Surface devices, and there have been a number of users who have solved hardware connectivity issues after replacing the special cable:
Accessories Designed for Surface
If the problem persists, in order not to continue to affect your use of the Samsung C34j79x Widescreen Monitor, we support and encourage you to contact your sales representative to request a refund.
We respect any of your opinions or decisions.
Best Regards.
Leo.L - Microsoft Community Support Specialist