This question Why Maharishi Bhrigu was so powerful? mention about Sage Bhrigu testing Lord Brahma, Shiva and Vishnu respectively.
I want to know what is the story?
The story of Sage Bhrigu testing the Trinity is described in Bhagvatam 10th Canto, Chapter 89.
Once a group of sages were performing yagna on the bank of river Saraswati. Then there a controversy arose that who is supreme among three chief deities. Then sage Bhrigu went to test them and ultimately Lord Vishnu was declared as Supreme by him.
SB 10.89.3 — To test how well Lord Brahmā was situated in the mode of goodness, Bhṛgu failed to bow down to him or glorify him with prayers. The lord became angry at him, inflamed into fury by his own passion.
SB 10.89.4 — Though anger toward his son was now rising within his heart, Lord Brahmā was able to subdue it by applying his intelligence, in the same way that fire is extinguished by its own product, water.
SB 10.89.5 — Bhṛgu then went to Mount Kailāsa. There Lord Śiva stood up and happily came forward to embrace his brother.
SB 10.89.6-7 — But Bhṛgu refused his embrace, telling him, “You are a deviant heretic.” At this Lord Śiva became angry, and his eyes burned ferociously. He raised his trident and was about to kill Bhṛgu when Goddess Devī fell at his feet and spoke some words to pacify him. Bhṛgu then left that place and went to Vaikuṇṭha, where Lord Janārdana resides.
SB 10.89.8-9 — There he went up to the Supreme Lord, who was lying with His head on the lap of His consort, Śrī, and kicked Him on the chest. The Lord then rose, along with Goddess Lakṣmī, as a sign of respect. Coming down from His bedstead, that supreme goal of all pure devotees bowed His head to the floor before the sage and told him, ‘Welcome, brāhmaṇa. Please sit in this chair and rest awhile. Kindly forgive us, dear master, for not noticing your arrival.’
SB 10.89.10-11 — “Please purify Me, My realm and the realms of the universal rulers devoted to Me by giving us the water that has washed your feet. This holy water is indeed what makes all places of pilgrimage sacred. Today, my lord, I have become the exclusive shelter of the goddess of fortune, Lakṣmī; she will consent to reside on My chest because your foot has rid it of sins.”
SB 10.89.12 — Śukadeva Gosvāmī said: Bhṛgu felt satisfied and delighted to hear the solemn words spoken by Lord Vaikuṇṭha. Overwhelmed with devotional ecstasy, he remained silent, his eyes brimming with tears.
SB 10.89.13 — O King, Bhṛgu then returned to the sacrificial arena of the wise Vedic authorities and described his entire experience to them.