Pi Desktop: max current output?

My limited understanding the Pi3B+ can only source the 2.5amps. Changes in the Pi4 only supported higher current capabilities. Even if you had a 3amp PS on the pi3's it was not used. I expect a 3amp PS would have less of a chance of sagging if under load but the Pi itself couldn't draw the current.

Now I don't have the technical reference to provide. This is anecdotal from what I have accumulated.

A Pi 3 doesn't need two fans. It's even possible to use fully passive heatsinking with the Pi 3 under full load, but that's difficult with that enclosure.

The fans will be consuming most power, not the RFXCom device or your other wireless device. If you can remove one fan, then there's a fair chance you may be ok in terms of power consumption, and it will still run at full speed.

shabaz thank you for your reply. Unfortunately this Element14 case is very tight and suffers from poor ventilation so a passive heatsink is not enough.

This Pi is critical as it runs all my home's domotics 24/7 incl heating, roller shutters and light.

I had to dig holes in the case and added 2 fans, one on the case side and one on the CPU heatsink to have the cpu temperature drop down to 55°C.
As I have a 3A PSU I'm going to investigate connecting the fans directly to the PSU, not to the Pi.

It is easy to suggest after the fact there are alternate solutions to cooling. I am impressed with the adafruit solution https://www.adafruit.com/product/4557 . Expensive is scaling and infrastructure intensive but it resolves airflow issues.

That's a good option, to connect directly to the PSU.

As a datapoint, I'm running a Pi 4 with no fan, in an enclosure with vents, and it runs very warm, but it's been stable for weeks in a room at 23 degrees ambient.

Pi 3's are easier to manage with some vents (could cut them in the enclosure) and a small heatsink, although if you expect to use all four cores, then it's more difficult (It can be solved with a larger heatsink and heat pipe for a fully passive solution, or with a single small fan + heatsink).

I've one Pi 3 in a capacitive touch display enclosure, relying solely on some vents, and several others just run with a heatsink. I don't believe they have throttled, but I expect them to do so if all four cores are used. My workloads don't usually involve that, your might be different.

Indeed. I"m looking for Element14 Pi Desktop RTC schematics as this would require to take the +5V after the on-off logical switch.
Not sure how this can be done

It's not an open hardware product as I understand, official schematics won't be available (it will require someone to manually trace them out, if they needed the schematic).