Ah! I see the cause for the confusion. The limit on attachments is indeed 3 per post. However, attachments are a subset of images. You can include up to 6 images using the [IMG] tag set (e.g., photobucket image links) in a post.
So it is actually possible to have 9 actual images in a single post - 3 attachments (thumbnails only) and 6 externally hosted images. If you include the attachments as enlarged images, then the limit of 6 comes into play even for the attachments.