How do I view my photos in the iCloud from my previous phones?

If you were using iCloud Photo Library on the previous phones and also turn it on in the new phone, the photos would sync to it and the photos on the phone would be the same as on iCloud, with the possible exception that some of them may be space-saving optimized versions if you have that option turned on. You view them in the Photos app on an iOS device. You can also use a computer (or force a mobile browser to access the desktop site) and login to www.icloud.com and go to the Photos app there. See: iCloud Photo Library - Apple Support

If you didn't have iCloud Photo Library turned on but do have an iCloud backup, you would need to use the backup of a previous phone to setup the new phone and that would replace all data (not just the photos) on the new phone with the data from iCloud. You cannot otherwise view the contents of the backup. iCloud: Restore or set up iOS devices from iCloud