1. Which Scriptures say Shiva is easily pleased than rest of Gods in Trinity?
Part 10 of Bhagavata Purana and Part 5 of Vishnu Purana
Lord Brahma, Lord Vishnu and Lord Shiva can each bestow or withhold favors, but whereas Lord Brahma and Lord Shiva are satisfied or angered very quickly (specially Lord Shiva), Lord Vishnu is not.
In this regard the Pauranik literature relates the following account:
A snap from 88th Chapter (Release of Lord Shiva from crisis) of part 10 of Bhagavata Purana:
Translation in English : Once the demon Vrakasura asked Narada which god was most quickly pleased, and Narada answered that Lord Shiva was.
2. Why is it said that other Gods are easy to please (specially Shiva) and not Vishnu?
Lord Krishna had said in Bhagavata Purana:
"O Yudhisthira, I take away all the wealth from those whom I bestow my grace. When they are poor, their relatives desert them. I even render their efforts useless when they try to earn money. Thus being unsuccessful repeatedly, the people develop an attitude of indifference for wealth. Then they begin to take shelter as my devotee and mingle with me. Only then, I bestow my full grace on them and they receive me as Sachchidanand Parabrahma. Thus, pleasing me through my worship is extremely difficult.
Hence, ordinary people give me up and worship other deities, which are in fact different forms of me.
Lord Shiva is Ashutosh and bestows his devotees quickly with grace. He gives wealth and other riches to his devotees. But once they get wealth, the devotees become despotic and even forget the God who had blessed them."
3. Such lines are there in each Purana of other Gods too
Yes, because most of the Puranas tell the conversation between two sages.
So, it depends on the faith, devotion and belief of a sage who tells the story.
For instance, take these lines from Ramcharitmanas by Goswami Tulasidasji:
जो संपति सिव रावनहि दीन्हि दिएँ दस माथ।
सोइ संपदा बिभीषनहि सकुचि दीन्ह रघुनाथ।।
Translation in English: This wealth was granted by Lord Shiva to Ravana after Ravana offered in sacrifice his own ten heads (i.e. had committed to serve Lord Shiva with all five sense organs and five organs of actions as preferred deity) and the same belonged to Vibhishana now which Raghunatha Shri Rama handed him with slight hesitation that it is not a sufficient reward for the divine virtues possessed by Vibhishana.