But in iMovie, only two of these movies show up in the iPhoto videos events. How do I get the third one to show up in iMovie?
Not sure, but try this: Close iMovie. From iPhoto, drag the AVI movie to the Desktop. This will create a copy on the Desktop. Now drag this file back into iPhoto. You will be asked if you wish to import a duplicate - click on Import. Now open iMovie - hopefully the newly imported AVI movie in iPhoto will now be recognised by iMovie, which will commence creating thumbnails. If desired, you can delete the original movie from iPhoto.
If this doesn't work, try converting the AVI file to another format. This can be done in either QuickTime 7 or QuickTime X, depending on the conversion required. Open the AVI file on your Desktop (the one dragged from iPhoto earlier) in QuickTime X. If the movie is 640 x 480 go to File>Save As and choose the format HD 480p. That will save the movie in H.264 format using 2 Mbits/s at up to 640 x 480 resolution. If your original movie is in some other resolution, you will probably be given other Save As options, so choose the most appropriate. If you have QuickTime 7, you can export using a much wider range of formats. However, the best to use are AIC (Apple Intermediate Codec) or the H.264 format - both work perfectly with iMovie '09.
Once you have the exported file (whether from QuickTime 7 or X), import it to iMovie. In iMovie's menu choose File>Import Movies. You should now be in business!
John