Since we cannot use plugins on WordPress.com, here are a few tips to help you manage your Media Library.
Images in the Media Library appear by upload date, with the newest at the top.
You can use the search box to find images, if you remember the file name, or use the dropdown menu which shows a date sort.
Next you can use the links at the top of the Media Library to find unattached images. These images may be appearing as headers or background images or used in your site's sidebar. It may also be possible to find images that you uploaded but which were not used in either a page or a post.
Lastly, the column headings in the Media Library are links which can help you sort your media.
Let me add a word of caution: Many a frustrated user has come through the forum after "tidying" their Media Library only to discover that the images they deleted were actually used in a post or page. Once deleted, the media is gone without any way to recover it other than uploading it again and reinserting it in the appropriate place.
If you are optimizing your images before uploading, that 3GB of space will last you a very, very long time. No need to delete anything really.