I love traps.
I also like thermostatic traps for tracing and most drip trap applications, as long as the subcooling of condensate can be done in the piping and not in the steam line for drip traps.
Since bimetallic traps ARE thermostatic traps, I assume in your question that you are comparing them to liquid filled type thermostatic traps.
The bimetallic traps are more robust and will hold up better if there is water hammer present. They also do not follow the steam curve as closely as the liquid filled traps. So for constant pressure applications like drips and tracers - which also have very small condensate loads - they generally work just fine.
For liquid filled traps, if they are true bellows types, these should also not have much problems at 3 barg. There is the capsule type that is an improvement on the bellow as there is less liquid fill and they are therefor more responsive, but the cost of these is usually a step up from the bellows, but probably still a bit less than the bimetallic.
So, without knowing more about the trap applications or your exact system, in general I would say at 3 barg without water hammer go with a quality bellows or capsule for drip and tracers, with the stipulation that the drip trap installation must take into account the space for condensate subcooling.
Joe Lambert