Hi Edder, I can imagine how disruptive a 3rd La Nina in a row could be! Regarding the La Nina intensity, this webpage is a good resource for getting an idea about the likelihood of various intensity possibilities. For example, it currently shows a 14% probability of the Nino 3.4 SST anomaly being less than -1.5C from November through February, so that means a 14% probability of a strong La Nina. It's difficult to say how that will change in the coming months, but we can expect the uncertainty about the upcoming winter to diminish, which means the probabilities for a small number of bins will increase and the rest will decrease. 

Regarding the accuracy of the predictions, I think this article is a good resource. Specifically, the bottom right panels in Figures 9 and 10 (two different metrics) give a concise summary. The main features are (1) forecasts targeting boreal fall and winter are rather accurate about 5-10 months in advance (depending on your error tolerance), and (2) forecasts targeting boreal spring and summer are much less accurate, due in large part to the spring predictability barrier.