I’m inclined against anything to do with higher tuples than pairs, honestly. If there’s a real demand for it at some point, we can certainly consider it, but pairs are vastly more common, and I’ve only ever wanted or seen people wanting the ‘a → (a, a)’ version in practice.
Generally speaking, I see ≥3-tuples as a sign of a missing data type, and I think I’m in likeminded company here.
If I really had my way, we’d make (,) a right-associative operator in both values and types, so (a, b, c) = (a, (b, c)), and accessor functions could work on any size of tuple.
I would also expect these functions to be called dup, dup3, dup4, &c. by analogy with other tuple functions using that convention (e.g. zip, zip3).