You can use an UPDATE FROM to accomplish this.

For example:

UPDATE Polling

SET Polling.X = VC.X, Polling.Y = VC.Y

FROM PollingCenters AS Polling

LEFT OUTER JOIN VotingCenters VC ON Polling.CodePlace = VC.CodePlace

This will...

No, the relationship will be between table3 and whatever table(s) you put after the ON clause.