On the one hand, if you're not in class, you can't participate in class, regardless of why you're not in class. On the other hand, if you're not in class, you also can't be on your phone or be disruptive in class.
To make it fair, you should get 0/3 for an unexcused absence and 0/0 for an excused absence. But that means that for grading, every student would have a different denominator, depending on how many excused absences they had.
For example, suppose there were 60 school days in a grading period. A student who was there every day would have total potential "work ethic" score of 180/180. A student who had 1 excused absence (i.e., was there for 59 days) would have a total potential "work ethic" score of 177/177. A student who had 10 excused absences would have a total potential "work ethic" score of 150/150. I don't think it would be hard to set up a spreadsheet to do that.