Darren Ellis, you could do this in a single smart list, but you would need to use advanced filter logic like
(Attended 1 AND
(Attended 2 AND
Since you only have three it won't be that hard.
If you have plan to have many webinars in the series and this will be an ongoing requirement, you could consider creating a new scoring field called, "Number of Webinars Attended" and then increment that score field +1 every time a lead gets a status of "Attended" in a webinar.
Then you can easily do a smart list to select leads where Number of Webinars Attended >=2.
If you're changing lead score based on attendance you could add the 'score was changed' filter and add constraints "Score Name" and "Min # of Times".
If not, you could export all three email lists and do it in excel.