Recently change the plugs. Says 3. But which one is 3?

Changed glow plug. It’s the big one with the connector this one if it’s 3rd from left. 
 

 

any ideas?

The expensive one obviously

I had a sort of “pinking” sound on our CR170 which I couldn’t identify, nor could the idiots at the local Skoda dealer. However, this completely disappeared and it regained its lovely progressive acceleration and smooth idle too, when all the glow plugs were replaced by a specialist last year. There’s no point doing one, as they’ll all be in the same condition and they’re relatively inexpensive. 

LReplaced them all. Same problem