Hi Steve,
Thank you for sharing your question. I understand that you wanna bring your old Surface 3 (the 128GB one you bought back in 2016) back to life, but you can’t find the original charger, and I am here to help.
Based on my knowledge and research, Surface 3 (not the Pro) charges with a micro USB, not the magnetic kind the Surface Pros use. Microsoft gave the Surface 3 a 13W charger back in the day, but folks have had better luck with a bit more power, like a 15W or 18W charger that does Power Delivery (PD). Just a heads up, not all micro USB cables and chargers are the same. Some of those off-brand ones don't give enough charge and slow.
Since Microsoft doesn’t sell the original Surface 3 charger anymore, your best bet is to grab a decent third-party micro USB charger that’s got the right power. Now, if your Surface 3's micro USB port is busted, you could try a Surface 3 Docking Station. It can charge it through its own special port.
May I also ask, is the micro USB port on your Surface 3 still good? Or is it damaged? Also, would you be OK with using a docking station if the port isn’t working right?
Let me know if you have more info or questions. I'm happy to help
Best Regards,
Noel