are the mountains in northern thailand part of the Himalayas?

Answer 1

The Thai Highlands are mountains in the north of Thailand that extend through Lao-PDR, Burma and China to link to the Himalayas, of which they may be considered foothills.

Answer 2

According to Wikipedia they "may be considered" foothills of the Himalayas but they are very far from the Himalayan range. If you look at a map you will see that there is a huge series of connected mountain ranges extending right across Asia from Turkey in the west through Iran, Afghanistan and Central Asia to Western China, Mongolia and Siberia in the north and to Pakistan, Kashmir, India, Nepal, Tibet, Southern China, Myanmar, Thailand, Laos and Vietnam in the south.

The Himalayas are the highest part of this followed by the Karakoram in Pakistan and Kashmir. The biggest and widest chunk is the Tibetan Plateau with its interconnected ranges (the Himalayas mark the southern boundary of it) but the central core of the ranges, from which they all could be said to stretch outwards, is the Pamir Range where Afghanistan, Tadzhikistan and Kyrgyzstan meet.

I don't think it's sensible to consider all these ranges as "foothills" of the Himalayas.

Answer 3

No.
The Himalayas is situated north of India, south-west of Tibet and Nepal.