Is 'pure' monotheism of more primitiveness and innateness than the Trinity is? I know that it is shocking for Trinitarians to ask this question, but it seems to me that monotheism, pure monotheism is more primitive. While Trinity is of less primitiveness and of less common sense.

Primitive and common sense means near to the pure heart or near to the original state or pureness or innocence or innateness.

So, the question is: is pure monotheism of less primitiveness and of less innateness than Trinity or Tritheism?.
It must first be stated that Trinitarianism is in fact monotheism. In Trinitarian belief there is only one God. However the nature of that one God is that he exists in three persons. But it is one of the fundamental doctrines of the Trinity that Trinitarianism believes in a single God.

Monotheism is not more primitive than polytheism.

It is a matter of historical record that ancient religions had many gods, and that monotheism exists only among more recent religions.

Is non-Trinitarian monotheism more "common sense" than the Trinity?

It depends what you mean by common-sense, but possibly. The concept of the Trinity is difficult to understand, and it appears to contradict our normal understanding of things.

However being less "common-sense" does not make it untrue. Many things in our world are not very "common-sense". It defies common sense that something can be both a wave and a particle, or that it can have an undefined position. Yet quantum physics tells us unquestionably that both are true. Quantum physics is less "common sense" than classical physics, but it is true.

Likewise it defies common-sense that something may get heavier as it goes faster, or that time moves at different rates for different things. Yet Relativity tells us that both are true. Relativity is less common-sense than Newtonian physics, but it is true.