If any single device, all by itself when all the other loads are off, is tripping a 20-amp breaker, then it certainly has a serious fault.
Far more likely, it is the sum of all the loads which is tripping the breaker; and no one of the devices is, alone, the culprit; no more than the last straw on top of the pounds of others is really responsible for breaking the camels' back. Devices with motors, such as an AC, are likely to draw a brief but very high load when they start up; laser printers do so too, for their heating element. This momentary draw, added to the normal draws of all the other devices, is exceeding the breaker's rating. (Breakers trip quickly with currents much above their rating, or slowly with currents slightly above their rating; either could be the case.)
Say the AC unit is drawing a steady 12 amps, which is the most it should be drawing if it's got a normal 15-amp plug, and you've got 3 3D printers each drawing 3 amps; well there's 21 amps, which will trip your breaker, not instantly, but soon. But none of the 3D printers is guilty of any excessive draw.
Since you've got a 20-amp circuit and call it AC instead of aircon, I'll guess you're in North America. The device you linked to is for the UK market and very different than anything that would be available (or legal) in the US or Canada. Likewise the small-value breakers at electronics shops are intended for installation within devices, not in line with the AC feed. I don't believe there is any mains breaker less than 15 amps in the US or Canada.
The only long-term solution (unless there is a real fault on one of the devices) is to have additional circuits installed and divide the loads among them. Very likely the AC is the prime candidate for its own circuit. But to figure out what you're dealing with, get a plug-in watt meter. A very common model is called Kill-A-Watt, but others are available too. The feature you're looking for is recording of the momentary maximum current, and a battery-backed memory so you can see the data even if the breaker trips, and I'm not 100% sure the kill-a-watt has that, so find one that does. Run it on each of the devices for a while, at least a full normal operating cycle, then plan your circuits for the worst case of all the machines drawing their maximum all at once, or plan your use of the machines to avoid any chance of exceeding 20 amps at any given moment.