An entry that could be the Surface Laptop 3 has appeared on Geekbench, and it has an AMD processor. Listed as OEMMO OEMMO Product Name EV2_1, it scores 3879 for single-core performance and 11894 for multi-core.
The OS is listed as a Windows 10 Enterprise Insider Preview, with 14312MB of DRR4 1198MHz SDRAM, and an AMD Ryzen SoC. Specifically, the chip listed is an AMD Ryzen 5 3550U with 4 cores and 8 threads.
The benchmark was discovered by Windows Latest and lines up with rumors that Microsoft will ditch Intel for its new device. However, it's worth noting that Geekbench scores can easily be faked. It happened with Pocophone F1 lite benchmarks, the iPhone 8, and countless others. A user can upload a real benchmark and change the name system properties with a bit of effort.
It's also worth noting that the previous rumors come from believable sources. Long-term Microsoft reporter Brad Sams said in June that sources told him the company is looking to diversify after poor SkyLake performance. This was added to by WinFuture in earlier in the month, who pointed to the Surface Laptop 3 specifically.
Rumors and benchmarks combined, it feels like a good indication of an AMD presence at the October 2nd Surface event. However, with the veracity of the benchmarks pending, we'll have to wait a week to know for sure.