You need to insert the disk to the newest expansion pack you have. Both pack will be played.

no, you do not need your Sims 3 disk, but you will need the disk for the expansion pack you want to install.

To add an expansion pack simply install the disk the same you would the original.

Unless you have an expansion pack installed you will need the base game disk in order to play. If you do have an expansion pack any of the disks can be used to play.

No you can't, you need the disk of the expansion pack to play the game.