Can I install gos on pixel 3 even if its EOL device

You can, but you should be aware that EOL devices are not secure. 3rd gen Pixels are also still on GrapheneOS 12.1, not 13, so it is not a complete representation of what GrapheneOS is like today.

Calyxos giving android 13to P3 devices why GOS is still on 12.1 pixel 3 is a wonderful device and I want to hold on to it a little more and I don't want to install calyx

Porting to 13 would cause a lot of regressions. This has been attempted with other devices in the past and has proved not to be worth it.

You will still get the occasional update on 3rd gen, but it's essentially on life support at this point.

While I'm sure your device works fine, it has been EOL for a very long time now, and it's incredibly insecure no matter what OS you install. I highly recommend getting a newer device if you care about security and privacy.

Thank you for your replay I think I have to move on to pixel 7 I guess

You can take a look at the project's current device recommendations here:

https://grapheneos.org/faq#recommended-devices

On a personal note, I would say that if you're looking for a budget option, go for the PIxel 6a. If not, a Pixel 7/7 Pro is a fine choice.

6th gen and above also have official support for 5 years instead of 3, so it's a phone you can keep for longer with proper security updates.