As per UTTARA KANDA of Valmiki Ramayana, Ravana had immunity from Lord Vishnu. Here is the story in short which was said by sage Agastya to Lord Rama:
Lanka was built by Vishvakarma. Malyavan, Sumali and Mali used to live in Lanka. Later they attacked Devas and Lord Vishnu came to fight with them. Lord Vishnu killed Mali. Seeing Mali slain, Sumali and Malyavan run away from battlefield towards Lanka along with their forces. But Lord Vishnu didn't stop killing Rakshas. Then, incapable of coping with Vishnu, leaving Lanka, they went to the nether regions (Rasatala) with their wives for dwelling there.
Later on coming to know that his daughter's sons, Ravana and his two brothers, got powerful boons form Lord Brahma, Sumali shed all his fear of Vishnu. He, with his ministers, Maricha, Prahasta, Virupaksha, Mahodara and other mighty Rakshasas came to see Ravana from Rasatala and said:
Dear Ravana! From fear of Vishnu, we are residing in Rasatala. Now we will all rally round you in our present prospirity and you will be the protector of all our race of Rakshasas from now on. (English translation source)
The above narration answers your question "Vishnu seems to think Ravana had immunity from him". Ravana had no immunity that he was incapable of being defeated by Lord Vishnu, only immunity he had was incapable of being slain by Lord Vishnu.
Coming to your next question, Was Ravana capable of being defeated by Trimurtis?
The answer is Yes. The birth name of Ravana was Dasanana. When in pride he lifted Kailash mountain, Lord Rudra pressed the mountain with his great toe and Dasanana could not bear this. He started crying and due to this reason he got the name Ravana. He accepted his defeat and started praying Lord Mahadeva, and thereat that Lord Mahadeva, pleased, set free the hands of Dasanana.
But Ravana was incapable of being slain due to the boon of Lord Brahma. Whenever someone wanted to kill Ravana, either Lord Brahma or Ravana's grand father Pulastya came to save him in order to keep Lord Brahma's words true. Lord Brahma saved him from Yama and from the Danavas of Rasatala (this is explained in this answer). His grandfather saved him from king Mandhata (this is explained in this answer) and Kartvirya Arjuna (this is explained in this answer).
Footnotes: In general the term Gods means all Devas including Trimurtis. But specific terms like Adityas or Vasus doesn't include Trimurtis and other type of Devas.