Why do I see three 'beams' when I look to a distant light source at dark?

From physical point of view:

• One reason is diffraction spike by the mounts (support vanes) of optical reflector of reflecting telescope. Four-fold mounts give four-fold diffraction pattern. However, three-fold mounts give six-fold diffraction pattern since we can see intensity but not the phase of the diffraction pattern. This can be reduced by other design.

• Photographers can add star effect filters (which contains etched gridlines) on their cameras for shooting Christmas lights and so on.

  

From medical point of view:

• Another reason may be due to astigmatism, glare or starbursting of someone's eye.

  

• See the link for the image here and vision simulations here and the causes of starbursting appears in this site.

Thanks to agweber's comment which pointed me this Youtube video, I found the right solution: The cause of this effect are the so called suture lines.

A sketch of the lens and these lines can be found here.

Another image plus an additional video is on this webpage (Better watch it only if you can stand medical pictures / videos)

There one can really see the Y-shaped suture lines. On that site, they also state that:

... sutures form because of how the lens epithelial cells proliferate during formation.

So, the effect is absolutely normal, and no deseases or abnormal defects are responsible for it.