This could be done with a daily backup script which performs an incremental rsync backup by using the --link-dest option, and deletes any backups older than 90 days. You don't need the find command or the rsync --delete option.
I am not sure this is what you asked for or what you need, but I'll tell you what it will do for you: For each day in the desired 90-day history, you will be able to access the files as they existed at the time of the daily backup. The backups could be saved in date-coded directories, for example, a directory called 2014-12-13 for today's backup, a directory called 2014-12-12 for yesterday's backup, and so on. Since you used the rsync --link-dest option, files which did not change will be hardlinked, which saves disk space; the same file will not be stored twice, but it will appear in multiple daily backup directories because of the hardlinks.
If this sounds like what you need, let me know, and I could give you more details about how to write the script.