Franco, you need to look at the line above that check:

	PyThreadState *check = PyGILState_GetThisThreadState();
	if (check && check->interp == newts->interp && check != newts)
		Py_FatalError("Invalid thread state for this thread");

PyGILState_GetThisThreadState returns the original tstate *for that
thread*.  What it's asserting is that, if there's a second tstate *in
that thread*, it must be in a different subinterpreter.

It doesn't prevent your second and third tstate from sharing the same
subinterpreter, but it probably should, as this check implies it's an
invariant.