There is no dedicated triple script runtime at this time, although treason is the codename given to the hypothetical reference runtime that we eventually plan to implement. It will likely be based on a fork of Fabrice Bellard's QuickJS.
Because of the general properties of triple scripts, however, a runtime itself implemented as a triple script is possible. This will be essential for prototyping and eventually be the basis of our ability to ship an officially blessed runtime for legacy systems (i.e., a way to approximate "running treason in the browser").