If you wonder how Yoga can be harmful, just type "yoga" into the search engine available on the red bar at the top of this page.
Yoga is used as a come-on by many cults to gain access to potential converts.
(There is a lot about that on this website).
Many cults offer Yoga classes; the religious stuff is slowly introduced to receptive students.
3HO teaches Kundalini Yoga, not Hatha Yoga.
A long time ago I was exposed to a very unethical East Indian "guru" through Hatha Yoga classes, however.
3HO is a religious organization run by Sikhs. 3HO stands for Happy, Healthy, Holy Organization. The deceased leader was Yogi Bhagan.
About Yogi Bhajan:
I do not know whether or not 3HO is a dangerous group.
I am familiar with other groups that use yoga as a way to recruit members.