By triangle I have in mind something where all sides must be length minimizing geodesics, i.e. geodesic segments. In particular the distances between their endpoints will be the length of the geodesic. Cf. this question.
Question: Assume $M$ is a connected and geodesically complete smooth Riemannian manifold. Then given points $A$, $B$, $C$, $A'$, $B'$, $C'$, and $d(A,B) = d(A',B')$, $d(B,C)=d(B',C')$, and $\angle (A,B,C) = \angle(A',B',C')$, can we conclude that $d(A,C)=d(A',C')$, $\angle(B,C,A)=\angle(B',C',A')$, and $\angle(C,A,B)=\angle(C',A',B')$?
Question 2: Same as the above but assume $M$ is additionally simply connected. (E.g. no tori.)
Because this is probably a basic and well-known result, a pointer to a reference would suffice.
Background/motivation/what I've considered so far: The SAS postulate is true in absolute geometry (i.e. Euclidean or hyperbolic), cf. this question. And apparently it is also true in spherical geometry using the correct notion of "triangle" or "side", i.e. length-minimizing geodesics, cf. this question.
So that covers all of the ($2$-dimensional, but the proofs probably don't depend on the dimension) Riemannian manifolds of constant curvature. But those are a very special class, so it's not clear to me that it would generalize.
There would seem to potentially be topological obstructions in general, i.e. due to the Riemannian manifold being mildly "pathological", hence my restriction to connected and geodesically complete (= complete metric space by Hopf-Rinow theorem) smooth Riemannian manifolds. (To be honest those are the only Riemannian manifolds I am interested in anyway.)
But even for "non-pathological" Riemannian manifolds, it still seems like the most possible obstruction to generalizing a result that holds for those with constant curvature is topological. E.g. with a torus, if the hypothetical missing thirds segment "passes through the donut hole" for $A$ and $C$, but not for $A'$ and $C'$, then I could imagine this failing.
But would that actually be a valid counterexample? And if so, is lack of simple connectedness the only possible obstruction?
A "global" topological obstruction (such as lack of simple connectedness) being the only possible obstruction seems conceivable to the extent that Riemannian manifolds are "locally" or "infinitesimally" Euclidean. But of course all of this is mostly hand-waving and speculation on my part.
Related questions:
Riemannian Triangles Third Side
Bounding angles in Riemannian triangles with bounded sides