Austrian, BMI, LOT Polish, SAS, Spanair, TAP Portugal, and USAirways all have 2 classes - Biz and Econ. Some have Econ +, but that hardly counts as a true '3 class' service.

The other 9 carriers - AC, Asiana, ANA, Thai, UA, AZ, LH, Varig, Singapore have 3 classes.

It was once rumored that a prerequisite for Star Alliance inclusion was having 3 classes, but this is clearly not the case and would not be a reason for CO to be barred. Rather, the fact that pretty much every CO destination is already served by a * carrier would be enough reason to not admit them. They wouldn't bring much to the table. USAirways rounded out the alliance nicely with more extensive East coast coverage, as well as a vast Caribbean network.