Just to clarify .....
..... do you want to make a "cylinder" whose cross-section is an equilateral triangle ?
If you do, then bear this in mind: the "included angle" at each corner of an equilateral triangle is 60 degrees .... I am sure you know that.
So the "excluded angle" at each corner is 120 degrees.
That would mean, if I understand your requirement correctly, that you want to tilt your saw blade to 60 degrees from the vertical position, because at each corner of the equilateral cross-section you will want a cut bevel at 30 degrees to the horizontal so that two bevelled pieces then combine to 60 degrees (included angle of the equilateral corner).
Adjusting your saw blade to that angle would be difficult or impossible, I would think.
So tilt the saw blade at 30 degrees from the vertical, and make a sloped base (to lay on the saw table) to give another 30 degrees.
(Exactly how you make and hold that sloped base I do not know.)
The edge of your work piece will then be bevelled at 60 degrees from the vertical. . (30 degrees from the horizontal)
When you put two sides of your desired "cylinder" together there will be a 60 degree included angle (which will show a cut bisecting each of the 60 degree included corners).
Anyway, I may be misunderstanding you, but that is my "conceptual" (not necessarily practical) solution to the problem.
Now, then, if you want a cylindrical ROD with the cross-section of an equilateral triangle, the process is quite a bit easier.
Are you actually making a pen ( ie. ROD ) ? . Size is what might limit you (and dictate the process) if that is the case.
But I would think that you could quite easily drill a triangular ROD on a lathe using a 3-jaw chuck in the head stock.
Of course I don't know what cross-sectional shape you require the cavity to be .... maybe it has to be of triangular cross-section also ?