I do not believe any of the boys are castrated but I could be wrong.
When Henry first arrived from Australia they began introductions to Ganuk pretty soon thereafter. Henry was very young and I don't think he's reached maturity yet even now (may be why Inukshuk has taken such a shine to Henry, he' not a threat yet). They have been pals for a couple years now.
Inukshuk has always been a social guy. He's lived off and on with Toronto's females Nikita and Aurora for years and always enjoyed the company of another bear. He's been in visual contact with Ganuk off and on for years as well when he moved back and forth between Toronto and Cochrane each year so it's not like they are total strangers.
Another thing that might help is that they have a number of habitats which allow the bears to be together when they want to and separate at other times. They have a massive enclosed lake exhibit the boys can enjoy at the same time and if they want to be no where near each other that's a realistic possibility.
I'm not sure if this helps as well but the facility has no females they need to compete over.
If you want to know more about the integration process and how they have successfully done it maybe you should reach out to Cochrane's facebook page. They seem to respond to questions quite a bit. It is an interesting process getting three large adult male polar bears together. If it's something that could be replicated with the right bears it is something that really might help with captive management.