Elements
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element
An element.
Just 3, as in the question. You cannot get simpler that that!
This is true not just in geometry but in every field of knowledge. You can define complicated concepts in terms of simpler ones, and those simpler ones in still simpler ones and so on. However, you will end up with a few terms which cannot be defined in terms of simpler concepts (without going into a circular definition). These terms must remain undefined.
Such a shape cannot exist in ordinary 3 dimensional space.