We don’t have a very strong opinion on the particular syntax in the team.
However, the proposed '''...''' seems quite reasonable in practice.
It has the least number of drawbacks and edge cases, it’s relatively easy to support and to comprehend when reading.
I first was leaning to the version with a single quote ". But it seems to introduce too many edge cases. In practice the overloading of ' semantics shouldn’t bring much cognitive load (again, compared to alternatives), especially when reading the code. What users will see is:
'''
SOME CONTENT
'''
all highlighted in green.
It will be hard to confuse it with a character 'A'.
As for other alternatives, it would be nice if the change was solely syntactic and did NOT require resolution/any compiler phases. So I would avoid solutions with some new special interpolators or macro methods.