I would not recommend printing the entire undersuit. Printable Flexible materials have come a long way, but they are still not nearly flexible enough for you to have the range of motion you would need.
Several folks will 3D print a neck seal in flexible materials like TPU, like
SSGLordBert, but once you get down to the Torso and limbs, you are going to want much more flexibility. The armor is also going to cover large sections of the body, so, you don't need to print the whole undersuit anyway.
What most folks who are going for an accurate as possible undersuit will do is to make "gaskets" where the details are only present in the gaps between the armor components, like at the elbows, knees, shoulders, and abdomen, and everywhere else is plain, moisture wicking material. It is common to do this in a latex or foam rubber material. The user will 3D print or sculpt he detail section, then make a mold, then cast it in the extremely flexible material and attach or glue that to a base compression or moisture wicking suit or material.
It's for the Mark VV/Infinite Chief, but
marinesniper did this process for his undersuit:
I am almost done printing the under suit off master chief infinite and I am in the process of body work on the mark 7 under suit I will cast both of these in latex rubber so they will be really flexible
www.405th.com
You can also explore the option of cutting the undersuit details out of thin EVA foam and gluing them directly to a set of compression style clothing, like
Spidermonkey60 has done for several suits.