Greetings

I have Kindle 3, the other night I had to factory reset it for a screen problem, the reset worked and back to normal.

But the device is no longer supported, therefore, unable to reload my purchased books. I can only copy non-drm books onto it now.

I have tried every possible suggestion out there and even tech support tried, I can no longer log back in onto the device. Any body have better luck than me?

At this stage of the game, I am not able to afford a new Kindle.
Remove the DRM from your Amazon purchases and sideload them.

You could probably get a new basic Kindle for about 100$. And it's still a lot better than the 12 years old Keyboard version. Or you could buy an used Kindle from Ebay.
DRM removal can work for me

100$ in my country is like 3 months salary
Do you have a mobile & SMS?
I read somewhere that to re-register some older models they send an SMS, so need your mobile number added to the account.

I've been able to register "out of support" old models. Not supported just means no updates and maybe no shopping via the ereader.
Can you manually register the device on the My Kindle Content and Device page? You could then send your books from that page to your Kindle.

Edit: it looks like Amazon has removed that option. There are several guides for doing a web based registration online, but I can't find the "register a kindle" page.
Yeah, that's no longer possible. Those guides are probably 10 years old.
Not since ages ago.
Download the latest firmware for the Kindle 3 and manually install it. The try registering. It's most likely that the firmware on it now is too old to register.
Scroll down here
https://www.amazon.com/gp/help/customer/display.html?nodeId=GKMQC26VQQMM8XSW
Tried all of the above suggestions, non have worked for me.