Aperture 3 won't open.

You need Aperture 3.6 -- what version do you have?

see Updating to Aperture 3.6 After the Release of Photos

LN

I see that it is no longer supported. How can I save the few photos I would like to save if I uninstalled and reinstalled?

do you want to continue using Aperture? Aperture is running on MacOS X 10.11.4, but it time to move on. Aperture has a few minor glitches like upside down thumbnails in list view. And it is no longer possible to order print products using Aperture.

You can update to Aperture 3.6 if you own an App Store version, as described in the user tip that Larry linked to. But if you just want to save photos from your Aperture Libraries, you can open any Aperture Library in Photos for Mac. Drag the library onto the Photos icon to create a migrated version. Photos will show all edits, albums, keywords, most of your Metadata, and you can export the photos or use them in the Media Browser from other applications.

See this Help: How Photos handles content and metadata from iPhoto and Aperture - Apple Support

How Aperture content and metadata migrate to Photos

Photos and organization:

• Photos and movies migrate without changes. Photos migrate with the adjustments and filters that you applied in Aperture. You can't change the adjustments in Photos, but you can revert to the original image.
• Projects and subfolders migrate to folders that are labeled "Aperture Projects" and "iPhoto Events" in the Albums view.
• Photos preserves albums.
• Photos preserves most Smart Albums. Smart Albums with selection criteria that Photos doesn't support usually migrate to Smart Albums with "(modified)" added to the Smart Album name. If Photos doesn't support any of the Smart Album criteria, it doesn't migrate the Smart Album.
• Photo books migrate to Albums.
• Photos preserves slideshows. If a slideshow theme isn't available in Photos, it uses the default Photos slideshow theme.
• Photos preserves Faces data.

Keywords and other metadata:

• Photos preserves keywords.
• Photos preserves user-defined titles. Images without user-defined titles show as "untitled" in Photos.
• Star ratings migrate as similar keywords in Photos, such as "1 Star," "2 Stars," and so on.
• Flagged images migrate with the keyword "Flagged" and appear in the "Flagged" Smart Album in the Albums view.
• Color labels migrate as keywords, such as "Green" and "Purple."
• Photos preserves captions.
• Most metadata is preserved in Photos. Some IPTC metadata, including Copyright, won't appear in Photos. This metadata is still associated with the image, and you'll still see it in other applications that show IPTC metadata, like iPhoto and Aperture.
• Custom metadata fields don't migrate to Photos.