Why are there three Gods (Brahma-Vishnu-Shiv) if there is only one God, as stated in the Vedas or Puranas?

 Om Namo Bhagavate Vasudevaya!

The answer is given in the Vishnu Purana.

ParaBrahman creates through Lord Brahma, maintains through Lord Vishnu, and destroys through Lord Shiva.

srishtisthithyanthakaraneem brahmavishnushivaathmikaam sa sajnaam yaathi bhagavaan eka eva janaardhanaha

Translation:

“In order to handle creation, sustenance, and destruction, the one and only Janaardhana takes the names of Brahma, Vishnu, and Maheswara.” - Vishnu Purana 1.2.66

The term ‘Trimurti’ literally means ‘three forms’ and refers to them being the three guna-avataras (i.e., the three expansions that oversee the cycle of creation, maintenance, and destruction).

Hence, in this context, they are qualitatively nondifferent from one another.

śrī-bhagavān uvāca
ahaṁ brahmā ca śarvaś ca jagataḥ kāraṇaṁ param ātmeśvara upadraṣṭā svayan-dṛg aviśeṣaṇaḥ

Translation:

“Lord Viṣṇu replied: Brahmā, Lord Śiva and I are the supreme cause of the material manifestation. I am the Supersoul, the self-sufficient witness. But impersonally there is no difference between Brahmā, Lord Śiva and Me.” - Srimad Bhagavatam 4.7.50

The reason the Trimurti exists is because it is believed that ParaBrahman never directly associates with the material realm to create, maintain, and destroy and acts indirectly through His expansions.

This is stated by Lord Brahma:

māyayāramamāṇasya na viyogas tayā saha

Translation:

“The Supreme Personality of Godhead never consorts with His illusory energy. Still, her connection is not entirely cut off from the Absolute Truth.” - Brahma Samhita 5.7

This is stated by Lord Vishnu:

ātma-māyāṁ samāviśya so ’haṁ guṇamayīṁ dvija sṛjan rakṣan haran viśvaṁ dadhre saṁjñāṁ kriyocitām

Translation:

“My dear Dakṣa Dvija, I am the original Personality of Godhead, but in order to create, maintain and annihilate this cosmic manifestation, I act through My material energy, and according to the different grades of activity, My representations are differently named.” - Srimad Bhagavatam 4.7.51

This is stated by Lord Shiva:

yam āhur asya sthiti-janma-saṁyamaṁ tribhir vihīnaṁ yam anantam ṛṣayaḥ

Translation:

“All the great sages accept the Lord as the source of creation, maintenance and destruction, although He actually has nothing to do with these activities. Therefore the Lord is called unlimited.” - Srimad Bhagavatam 5.17.21

Hare Krishna

According to all Puranas who is more powerful? Asuras or Rakshasa?

 Asuras are easily more powerful due to their divine origin. They are naturally more powerful without plot armour like boons from gods .

Let's take demon vritra for example. He was so powerful that he was devouring the RTA cosmic waters of the universe . After killing vritrasur Indra restored the universe

• yad indrāhan prathamajām ahīnām
• ān māyinām aminā protā māyāḥ |
• āt sūryaṃ janayan dyām uṣāsaṃ
• tad īt nuṃ śatruṃ na kilā vivitse || 1.32.4 ||
• Literal
• “When, O Indra, you slew the firstborn of the serpents,
• you destroyed the magic/powers of the magicians.
• Then, generating Sun, Heaven and Dawn,
• you found no enemy indeed to withstand you.”

दि॒वो न तुभ्य॒मन्वि॑न्द्र स॒त्रासु॒र्यं॑ दे॒वेभि॑र्धायि॒ विश्व॑म् ।

अहि॒ं यद् वृ॒त्रम॒पो व॑व्रि॒वां॒सं हन्नृ॑जीषि॒न् विष्णु॑ना सचा॒नः ॥

It took the combined might of universe striding Vishnu and king of gods Indra to defeat him as explained in Rigveda

• uta mātā mahiṣam anv avenad amī tvā jahati putra devāḥ |
• athābravīd vṛtram indro haniṣyan sakhe viṣṇo vitaraṃ vi kramasva ||
• “And the mother looked upon the mighty one:
• ‘These gods are abandoning you, O son.’
• Then Indra, about to slay Vṛtra, said:
• ‘O friend Viṣṇu, stride out farther/wider.’”

विष्णुना सचानः

viṣṇunā sacānaḥ

“accompanied by Viṣṇu” — RV 6.20.2.

Similar example would be Madhu and kaitabha from ithihasas whose flesh was used to forge the planet Earth and mountains

• They were Madhu and Kaitabha and their bones covered everything and constituted mountains. The earth was formed from their fat

Rakshasas were human hybrids of ghandravas and humans . They are cannibals but nowhere possessed the power on the same cosmic scale as Asuras . Except Ravana and Kumbhakarna, who were descendants of saptarishis