I know of Shaivism, Vaishnavism, Shaktism only. Please increase my knowledge by listing all the sects which existed before 'Hinduism' arose, and who were their Chief Gods/ Goddesses. I am aware that these sects have now almost vanished, but I still want to know more about them
Let us understand few terms very clearly - before I delve into the answer to this question.
The word Hindu is merely indicative of group / civilization of people - who used to live near / beside / beyond and south of Sindhu (English: Indus) river - during ancient times. Since the people from other civilizations - were unable to pronounce the word "Sindhu" (pronunciation of S - as such) - and used to pronounce "Hindu" (S was being pronounced as H). So the people in modern Indian sub-continent - were addressed as "Hindu" i.e. people living in that (of river Sindhu) area.
Therefore, speaking technically - there is no such things called Hinduism.
The word Hindu - is NOT found in ANY of religious books, writings, scriptures etc.
Since (in modern terms) - most of literature (secular or religious) - composed during last almost three millennia - was composed either in Sanskrit, or Pali or Prakrit or many other languages / scripts - were as well as currently spoken in India. So, in these works also we do not find the word Hindu or Hinduism (or their synonyms in different languages).
On contrary, what we know and say that we are people of dharma. And most correct alternative usage even today will be Sanatana Dharma. Here word Sanatana means Eternal and Dharma means either law or basic nature etc.
The English word religion has Sanskrit synonym panth which in English means - either sect or path.
Have said this, if we still want to know - how many religions does exist today or have come into existence since past 5-6 millennia in this part of World? Then simplest guesstimate would be around 300! Yes...I mean it
Hinduism (as a social or cultural identity) - includes very very diversified - and even mutually contrary beliefs or faiths.
Let me further elaborate by comparing with other (mostly semitic religions).....
For example, if I do NOT believe in Jesus Christ and / or The Bible - then definitely I can NOT be called as a person belonging to Christianity. Isn't it?
But being a Hindu - I am (rather) free to believe in one god (Monotheism) or many gods (polytheism) or in no gods (atheism) etc. Or even I could be an agnostic? But I am still a Hindu. Similarly, I may believe in Vedas, or Puranas, or Tantra or dozens of other schools of philosophies existing right now - but I am still a Hindu.
Furthermore, I may believe or know or pray to Rama or Shiva or Krishna or Surya or Ganapati or other gods (and there are 100s of gods / semi-gods / demi-gods) - I am still a Hindu.
After this long explanation (as an attempt to bring the clarity on words like Hindu or Hinduism - I would conclude with my crisp answer to the question below:
All above mentioned sects (panths or religions) have been part of what we call Sanatana Dharma. And since in modern terminology and usage - Hinduism which is a misnomer for Sanatana Dharma, therefore all these sects have been co-existing since past 3 millennia (at least).