Those are the correct DLLs. I just tested on my end using Unity 2017.3.0f3 using the same package and everything is working fine. So we have some mystery on our hands.
Although you have already done this. Please try creating another new Empty project. Then import the TextMesh Pro package from the link for 2017.3 found in this thread and see if you get the same results.
If possible, can you create a video showing the steps you are taking to create the Dropdown from the Create - UI - TextMeshPro - Dropdown menu? I realize this might appear silly but like I said we have some mystery on our hands so let’s look at everything.