Q:

What is the meaning of the 9 sacred threads in janoi?

I am soon going to undertake Upanayana(Yagnopavitam) and I was researching and I couldn't find any straight answer to my question and I would be grateful for any help.

A:

The sacred thread is not considered just as an ordinary thread. Before wearing one, it has to undergo a process called Samskara, which is a purificatory ritual that is aimed at invoking the presence of various deities on the 9 threads and the 3 knots.
The relevant Mantras can be found on page 55 of the prayoga book called "Nitya Karma Puja Prakasha".

Transliteration
Prathamotantau Om omkaram avahayami | Dvitiyatantau agni avahayami | 
  ..... Prathamagranthau Om Brahmane namah, Brahmanam avahayami |
  Dvitiyagranthau Om Vishnave namah, Vishum avahayami | Trityagranthau
  Om Rudraya namah, Rudram avahayami ||

So, what these mantras say is that in the Yajnopavita there are 9 interwined threads and 3 knots. In the 9 threads Om, Agni, Sarpa, Soma .. Surya, Viswadevas are invoked.
And, on the 3 knots (Granthi), the Trinities are invoked viz - Brahma, Vishnu and Rudra in that order.
Translations of the bold portions are as follows:

Prathamagranthau Om Brahmane namah, Brahmanam avayami - In the first
  knot, Om salutations to Lord Brahma, I am inviting the presence of
  Brahma on this knot.

Similarly for the other two knots.
Therefore, the 3 knots represent the Trinities - Brahma, Vishnu and Rudra and their presence on the sacred thread.

