There exists 3 types of Gunas which are the base of Universe and modes of existence. Complicated concepts these are indeed, so here I have simplified them so you can understand it easily.

These originate from the concept of Trigunamaye Mahamaya.

Although Tridev are considered Nirgun but these Gunas have been associated with them.

Sattwa Guna : Understand this as positive energy. Sattwa mind is calm, present that is aware, happy and free of desire. This Guna is about purity and Harmony. Sattwa Guna is associated with Narayan.

Tamas Guna : Consider this as Negative energy not evil, although it leads to evilness the guna itself is not evil. This Guna is about, ignorance, chaos and destruction. Tamas Guna is associated with Mahadev.

Rajas Guna : See it as a mixture of both the Sattwa and Tamas Guna. Rajas mind is passionate, aggressive, frustrated and excited. This Guna is about activity, passion and manipulation. Rajas Guna is associated with Brahmadev.

Tamas and Rajas are necessary for the functioning of this world and Sattwa is there to create balance between them.

For eg. Working (Rajas) is important but so is resting (Tamas). But too much work can exhaust you and too much resting can make you lazy. Thus there is a need to realize (Sattwa) how much should we work and how much should we rest.

These three gunas are present in the material energy, and our mind is made from the same energy. Hence, all the three gunas are present in our mind as well. They can be compared to three wrestlers competing with each other to gain dominance over one's mind.

These Gunas appear in Charak samhita which is Vedic psychology and also in Samakhya philosophy.

It depends upon the external environment, the internal contemplation and the morals, and that's how one or the other guna begins to dominate.

As Shree Krishna said in Geeta chapter 14, verse 10 :

Rajas tamaśh chābhibhūya sattvaṁ bhavati bhārata
Rajaḥ sattvaṁ tamaśh chaiva tamaḥ sattvaṁ rajas tathā

Sometimes goodness (sattva) prevails over passion (rajas) and ignorance (tamas), O scion of Bharat.

Sometimes passion (rajas) dominates goodness (sattva) and ignorance (tamas).

And at other times ignorance (tamas) overcomes goodness (sattva) and passion (rajas).

So now as we have understood the three Gunas we shall start the story.....