I was wondering what the correct process was for this. Lets say i have 3 disks and i have a share that uses all disks. Now lets say i want to move that share to only use 1 disk. If i change it in the settings will it automatically move all the files to that single disk or will it just put new files on that disk?

No, it won't move files => any share restrictions r.e. which drives to include only apply to WRITES of new data.    Any disks that have content for that share already on them will still be included.    If you want to move all the content to another drive, you have to do that manually -- and if you do that, be sure to read about and AVOID the "user share copy bug" ... which can result in total data loss.