This is where most people fail. The Tenda N3 has specific hardware revisions (e.g., V1, V2, V3). Installing firmware for V2 on a V1 router will destroy it.
... The router’s lights went dark for a terrifying second. tenda n3 firmware update verified
This article provides a deep dive into everything you need to know about finding, verifying, and installing the correct firmware for your Tenda N3 router. This is where most people fail