There is a common mis-perception that the 3rd wire on 3-pin fans is always speed control and the 4th wire on 4-pin fans is always speed monitoring, because that is usually how PCs use 3- and 4-wire fans. That isn't the case and even if it was, there are many options just for speed monitoring. To get an idea of what sort of combinations there, look at this Nidec PDF. This is one reason I don't buy "modder-class" fans - I want fans with detailed datasheets, not (at best) an airflow graph or two.
- If I boot with a 4-pin PWM fan in the fan 3 position (closest to PSU), temperature monitoring always works, even with two 3-pins also connected
- If I then swap this 4-pin fan for a 3-pin while the switch is running, temp monitoring continues to work from that point onwards.
- It breaks again if the next power cycle has 3 x 3-pins again.
As someone else pointed out, fan speed monitoring and temperature sensing are probably running over a single bus. The switch was designed with speed-monitoring fans, so by definition if no fans are reporting a speed signal it is more likely that there is a single failure affecting that bus than all 3 fans failing that way. The hardware and software designers didn't forsee users who wanted to change out all the fans for incompatible ones.