How to trigger several cameras from a microcontroller
It's so sensitive because you don't have a reliable ground, so any interference, ground loop etc will trigger it.

The absolute best solution is to use opto-isolators. They guarantee no ground loop and other troubles and shocks. But if you want to try connecting things together...

I strongly suggest that you connect the ground (0 V) wires to the microcontroller ground.

Then connect each wire which floats at 5 V to one microcontroller pin. Now configure the pin as an input, no pull-up or pull-down. It should not trigger the camera. Then, when you want to trigger it, set the pin as an output and pull it low.

Look at your microcontroller datasheet, it is almost certain to have a high-Z mode, sometimes also called tri-state. If not, it might work to just keep the pin high.

It would be safer to use a 10 k resistor in series with the 5 V wire, but it might work ok without it.