Suppose that ABC and PQR are h-triangles.
Then there is a hyperbolic transformation t which maps
A to P,
B to B' on the h-line PQ, on the same side of P as Q, and
C to C' on the same side of the h-line as R.
Note that, as t preserves angle and hyperbolic distance,
If h-triangles ABC and PQR have
Let t be the transformation implied by the Basic Strategy.
Then B' lies on the h-line PQ.
By the Basic Strategy, d(A,B) = d(P,B').
By (2), d(A,B) = d(P,Q), so d(P,B') = d(P,Q), and hence B' = Q.
By the Basic Strategy, <BAC = <QPC'.
By (1), <BAC = <QPR, so C' lies on PR.
By the Basic Strategy, <ABC = <PQC'.
By (3), <ABC = <PQR, so C' lies on QR.
Thus C' lies on PR and on QR.
But the h-lines PR and QR meet only at R, so C' = R.
Thus t maps ABC to PQR, so the h-triangles are h-congruent.