Iphone not backing up

When i log to iclould i have more than 3 gb space left but my phone keeps telling me that it could not back up due to lack of space since 17 weeks

Your current backup may need more storage than 3GB, if you installed new apps with large data. What has changed recently?

And i have disabled back-up for photos.

There is no separate backup for photos. Do you mean, you disabled iCloud Photos (or iCloud Photos Library)? Disabling iCloud Photos will not immediately free the cloud storage. The photos are kept in iCloud for month, so you can recover them, unless you deleted all photos from your iPhone and emptied Recently Deleted, before you turned off iCloud Photos.

And after you disable iCloud Photos, the device backup of your iPhone will now include your photos. As long as you used iCloud Photos, the backups will have been small and needed less storage, because the photos have not been included in the backup. The required storage for the backup may now be much larger, if you have many photos on your iPhone.

See: About backups for iOS devices - Apple Support

iCloud backups include nearly all data and settings stored on your device. iCloud backups don't include:

Data that's already stored in iCloud, like Contacts, Calendars, Notes, iCloud Photos, iMessages, Voice Memos, text (SMS) and multimedia (MMS) messages, and Health data*

Data stored in other cloud services, like Gmail and Exchange mail

Apple Mail data

Apple Pay information and settings

Face ID or Touch ID settings

iCloud Music Library and App Store content (If it's still available in the iTunes, App, or Apple Books store, you can tap to re-download your already purchased content.)