Bear in mind that when calculating the best third place team, points gained against the bottom side are deducted if you're playing in a six-team group.
So we could finish third, with more points than any other team in third, but still not qualify automatically.
ie if we finish a point higher than Ukraine, but "lose" six points for beating Gibraltar twice, whereas they only lose four points because they had a victory and a draw against Luxembourg.
There's so many variables that I think it's going to be almost impossible to work out the best third-placed side in advance.