In practice, there is very little moderator activity which needs split-second response time from moderators. We do not request nor expect moderators to keep constant vigil over the site at all times. For that reason, there is little practical value to spreading three moderators across the thirds of the globe.
Some moderators cannot moderate while working; some only use the site while working. Some are night owls, while others not. So everyone has their own "peak contribution" timezone, independent of where they live. There will be a statistical tendency for moderators to reside in the timezones where most of the activity comes from — more users, more likelihood moderators will be from there. Makes sense. So, I see no need to divvy up the moderators in this fashion.
I have nominated about 140 pro tem moderators and timezone coverage has never been an issue.