Can iClone 8 be used together with CC3, OR can iClone 7 be used together with CC4?

No. Only the same generation software can be synced together. For instance, CC3 can transfer character to iClone 7, or CC4 can transfer character to iClone 8.

Take note that iClone 8 and CC4 are backwards compatible, meaning that iClone 8 can read iClone 7 projects.

Also, the Content Manager in iClone 8 and CC4 can read contents which were created using iClone 7and CC3.

Content Sharing with iClone 8 and Character Creator 4: