This is an image from a TForm where I have a TEdit at top and a TComboBox at down.

As you can see, the TEdit does not have the classic 3D border of the Windows controls default theme. That’s because I set the Ctl3D property of that component to False. Then you see that the TComboBox has it’s normal 3D border, but in this case I also set the Ctl3D property of that component to False, but it continues to show the 3D border.
It seems to be a Delphi bug at development level. How could I fix that at code?
After testing RRUZ answer, BevelKind=bkFlat, this came:

I do not know why it was so different… and bizarre. :-/
You can remove the 3D border setting the
BevelKindproperty of theTComboBoxtobkFlat.