As Suzanne noted, you cannot freely rotate a text box in Word.
However, if you are comfortable working in PowerPoint, you can. Once you
have your triangle all set up as you want it, you can zoom in on the triangle
and do a Print Screen, paste the image into Word, crop it as need using the
cropping tool on the Picture toolbar, use Format> Picture, and on the Picture
tab select Compress and use the default settings (this gets rid of everything
you have cropped from the picture, reducing the size of the graphic). The
only drawbacks of this method are that if you want to change your picture,
you need to repeat the process and your image quality may be diminished.
Hope this helps.