When I look at the diagram at subaruparts.com
http://www.subaruparts.com/diag/?mod...ategory=440A-1
The two front sensors (part no. 22690) are conjointly listed as No. 1, while the third sensor, at the converter, is listed as No. 2. It'd just be a guess, but, I'd say that the one at the converter would be number 3, in real terms. Driver's side would probably be number one, while pass'rs side would be number 2. Again, that's just a guess.