usually at or on the third stroke means time, for instance when you you phone a number to get the exact time ('On the third stroke it will be 11.31 and 15 seconds') - so he's saying that on the third stroke he doesn't know what time it will be, which I guess is a joke. But if he has been talking about illness elsewhere then maybe he is combining both meanings - which is also the sort of thing poetry does. So that could suggest the future is uncertain when you've had three strokes, and he's expressing it in terms of a telephone message about the stroke of a clock.