This was a tough one to evaluate, but I would say that the spot the third player (orange) just put his settlement on was the best choice for him. From the advice I've given so far you probably noticed that this site was good because it had a total of 11 dots, though it was also bad because it has ore and wood, which don't work together at all to build anything. Below I will list a couple of other pieces of advice to take into consideration when placing a settlement which make this the best spot for player three.
1) Expansion opportunities - The biggest reason I think this was the best spot for player three was the 11 dots, which is more than any other spot. However, what solidifies this spot as the best in my mind is the amount of decent spots around it. Right next door in all three directions there are spots with six to eight dots. Once the game starts, all the good spots have been taken, so anything with seven or more dots is a great place to expand to. Most starting settlement placement sites have only one or two nearby spots with such potential, though this location has three, and a fourth is a 3:1 port with five dots. So, after the first two or three settlements are on the board, you should pay attention to what spots are near a settlement you put down which you plan to expand to. However, keep in mind that if a spot is too good, someone else will probably put one of their starting settlements on it. Right now I think the third player is running that risk on at least one of his nearby expansion spots, but we'll see what happens.
2) Resource Rarity - The second reason this is a great place for player 3 to settle is that it provides great access to two resources which are going to be pretty uncommon in this game: ore and sheep. As noted in the first observation of this board, those two resources are going to be very limited in supply. So, any player with easy access to either will be able to use them to get better trades. Having good access to both makes even better trading opportunities which will help to make up for the lack of any good combinations of the three resources this settlement site provides. (That ore-lumber combo will be especially frustrating later in the game, when player three will have a lot of both and be unable to use them).
Phew, now we have the toughest decision in this early settlement placement done with. Next is another difficult decision, but this time for the fourth player. Player 4 is stuck with two mediocre settlement placements, but has the amazing advantage of being able to make sure they work together well. Considering all of the advice given so far, what two spots would you pick?