That leaves approximately 227,250 fires since the Tesla has been introduced, of which Teslas are known to be responsible for three. Or 0.000013% of all car fires in the relevant time.
That said, the other test might be more relevant, as there were more actual Teslas on the road in the past 140 days, rather than at day zero of launch. I don't have the figures to calculate, say, fires/#miles driven.
As to the point about them all being new cars, well, new cars are safer. If safety is what we're after then yes, a modern car is a good thing. If you're saying that the safety will decay as they age, well, that's reasonable but only time will allow us to correct our calculations there.
That might mean that we need more time to make a full determinations, but that's not a bad thing. This tells us about their current safety which is what's relevant now.