I have heard about these traditions: Vajrayana, Mahayana, and Hinayana. In today's world, Buddhist people dwell in South Asia and East Asia. Who among them follows each particular sect? Does anybody follow more than one of them?
In today's world Buddhist people dwell in South Asia and East Asia . Who among themselves follows particular sect.
Wikipedia's Schools of Buddhism article has a map.
Does anybody follows more than one of them?
I'm not sure that it makes sense to talk of "following a sect". If you think of them as "schools" then, yes, I'm pretty sure that there are some (or many?) people who learn from more than one school.
For example the Pali canon could be associated with the Theravada school, but it is studied by people who also learn from other (e.g. Tibetan or Zen) teachers.
I can't tell you how wide-spread that practice is geographically, but there are certainly some users like that on this site.
Incidentally, please refer to this topic: How should we police the use of the word Hinayana?
Also, One Vehicle for Peace (referenced above) says,
It is universally accepted by scholars that the terms Hinayana and Mahayana are later inventions by Mahayanists. Theravada cannot be included in either of these divisions. Historically speaking Theravada already existed long before these two terms came into being. etc.
The Wikipedia map I referenced shows Theravada, Mahayana, and Vajrayana (not Hinayana, Mahayana, and Vajrayana) as you wrote in the question.
Dhammadhatu's answer is about Hinayana ("individual enlightenment vehicle") since that's what you asked about -- it's one of the yanas which are described in Mahayana doctrine. But maybe (since you asked about different schools in different geographical places) you intended to ask instead about Theravada (which I think of as "old school" or "original school" or literally "school of the elder monks").
Hinayana (individual enlightenment vehicle) practitioners practise only Hinayana; i.e, the realisation of not-self (anatta) & emptiness (sunnata); which they will also teach to others who they regard as suitably predisposed.
Mahayana practitioners (developing 'bodhicitta'- the wish to attain enlightenment motivated by great compassion for all sentient beings) must practise both Hinayana (to realise emptiness) and Mahayana (altruistic intention).
Vajrayana (tantric) practitioners (developing the qualities of a deity or god) must practise Hinayana, Mahayana & Vajrayana because, if the Hinayana path is not realised, their tantric Vajrayana practise of self-deification will become egotistical & thus delusional.
The above was taught to me by a fully ordained Tibetan Vajrayana monk & geshe.
For me, Hinayana practitioners only practise Hinayana because they do not believe Mahayana (saving all sentiment beings & being reborn as a Bodhisatva for many future lives) is possible. Instead, they only help those who they believe Buddhism can help (which is not all people but only those suitably predisposed).