Hi,
Given the age of the device and the fact that it has never been used, there’s a fair chance that the battery has failed and is beyond recovery and needs to be replaced.
Here’s a link to the ifixit [guide|29844] guide.
Here’s a link to a [https://www.ebay.com/itm/170-1032-01-GP-S10-346392-0100-S11GTSF01A-Battery-for-Kindle-3-Keyboard-D00901-/251969615101|supplier] of a replacement battery for a Kindle 3. The link is also on the battery replacement guide listed under ''parts.''