My opinion is that the 3 foot rule applies whether you are in a car or on a bike. In other words none of us likes to have someone go past really close because it would mean that if we had swerved a little, for any reason, it would have resulted in an accident including most likely some broken bones and/or bike damage and no riding for a week or two. So if we use the old "do unto others....." rule - you should only pass someone closer than 3 ft, if YOU would like someone to pass YOU closer than 3 ft.