Here's a Forbes article that briefly mentioned it, but it was written after the Kyrie trade so a lot of it is written in past tense.
A bit hard to go back and find all the sources cause if you Google "Celtics Isaiah Thomas 3 year contract" or any variation of it, it brings up more recent articles talking about his career post-Celtics. Also doesn't help that there was never a Woj bomb or anything, since the offer never officially was made since IT had already laughed at the contract offer amount rumored/reported. Trying to find a couple way back on Google.
Here's one from NBA.com that specifically mentions 4 months before free agency he could have been signing a contract worth $100 million dollars.