There is no standard as far as I know. The middle of the three pendants would be centered over the island and the outer pendants equal distance on either side. I would say split the space to either end. We always visually hold up something in the space to make sure. Have you opened up your ceiling to make sure that you have the flexibility to put them where you want? Generally the ceiling joists will dictate what you ultimately end up doing.

How wide is the island? Is there anything on the island like a sink, or is it just there to get in the way? If you do actually use it for something, are you more likely to use it from the end where it might be more convenient to set things as you get them out of the refrigerator, or would you move everything to the middle to work? What they will look like with the cabinets behind them is also a consideration. Sometimes, keeping them in a little further from the ends feels better if you like them to be lower.

Thank you for the comments. We went ahead and had them placed equally apart - fixtures not installed yet, but I think its going to look great. Its just a counter, mostly for sitting and setting stuff on, so we're good on how much light we'll have here.