Intel 12th GEN CPU in Ubuntu 18.04 LTC
I want to make a computer composed by Intel 12th GEN CPU(I9-12900), and need to install Ubuntu 18.04 version because of my special software.(only hardware upgrade) Will this computer work properly? (all my searching shows that 12th Gen causes problem in Ubuntu 20.04.) additionally, for the VGA, RTX 3060 or higher level Graphic card could be worked on this system?
Let's break this down:
Q. Will this computer work?
Yes.
Q. Will this computer work properly?
This depends on your definition of "properly". If you want to use all the functions and features of the CPU, including its differential cores, then no. If you want to use the computer as a general purpose device that views YouTube and plays Sudoku, then sure. However, you have stated that you plan on using a piece of software that insists on Ubuntu 18.04, which already smells of "Legacy Nightmare" ...
Q. Additionally, for the VGA, RTX 3060 or higher level Graphic card could be worked on this system?
This depends on your motherboard, not the OS. You may also want to update your terminology. VGA (Virtual Graphics Adapter) is not really how one refers to the graphics subsystem anymore. You may just want to say "Will an RTX 3060 work with this CPU/Motherboard/etc.?". To answer the question, though, the OS does not have a system-level issue with the recent RTX models. Be sure to use a proper driver, and you'll be fine.
As an aside ...
If you can install your legacy software while using a Live USB (or DVD), then you will be able to answer many of these questions for yourself. It's really hard for people who are not you to determine whether something works properly or not, as everyone has their own criteria for "proper". If you are trying to determine compatibility before buying, asking on a site like this will not give you a satisfactory answer. If you are trying to determine compatibility before replacing Windows (or another pre-installed OS) with Ubuntu, then the Live USB method will give you a better answer than anyone else.