Hi @Adrian Van Brussel,
Welcome to Microsoft Q&A forum.
Thank you for reaching out regarding a replacement keyboard for your Surface Pro 3. Since this model belongs to an earlier generation of the Surface family, finding an original Type Cover 3 can be a challenge, but you actually have several excellent options due to the cross-compatibility of the Surface lineup.
1/ The best official choice is a Type Cover designed for newer models. Type Covers for the Surface Pro 4, 5, 6, 7, and 7+ are fully compatible with the Surface Pro 3. These newer versions offer a significantly improved trackpad and a more comfortable typing experience compared to the original Pro 3 version. You can often find these as "Certified Refurbished" or "Open Box" on major online marketplaces like Amazon, eBay, or Walmart.
2/ If you are looking for a more budget-friendly option, brands such as Fintie, MoKo, or Arteck produce keyboards specifically for the Surface Pro 3. Some of these connect via Bluetooth rather than the physical pins, allowing you to use the keyboard even when it is detached from the tablet.
Note: please avoid keyboards made for the Surface Pro 8, 9, 10, 11, or Pro X. These use the "Surface Pro Signature" connector, which is physically different and will not fit your Surface Pro 3.
Final Recommendation
For the best balance of quality and fit, I recommend searching for a Surface Pro Type Cover (designed for Pro 4 through 7+). It fits the Pro 3 perfectly and provides a more modern feel.
I hope this helps you find the right keyboard for your device. If you have any further questions or need assistance with anything else, feel free to let me know.