2009 Nissan Altima P0725, P0826, and P3431 Codes: Causes and Diagnosis Guide

My 2009 Nissan Altima has recently triggered three diagnostic trouble codes simultaneously: P0725, P0820, and P3431. This is the first time in over 109,000 miles that the check engine light has come on, which suggests I've been fortunate so far. I'm trying to understand how these three codes are related and where to begin troubleshooting. P0725 refers to a malfunction in the vehicle speed sensor located at the rear of the engine. Online resources and YouTube videos suggest this sensor can be replaced, and it's often linked with transmission issues. P0826 indicates a problem with the up/down shift switch circuit, which is part of the transmission control system. P3431 points to an issue with the exhaust valve actuator on cylinder 4C (cylinder 4, control side). I understand that P0725 and P0826 are both related to transmission operation and could be connected through a shared sensor or wiring fault. However, I'm unsure how P3431 — which relates to the exhaust valve actuator — fits into this picture. Are these three issues truly independent, or is there a deeper underlying cause? What should I check first when diagnosing all three codes at once?