Chandra, pyromaster ultimate

[Crossed]

Chandra, pyromaster

−7: Exile the top ten cards of your library. Choose an instant or sorcery card exiled this way and copy it three times. You may cast the copies without paying their mana costs.

Do I have to cast all of the copies? Can I choose to only cast one? Say I choose a shrapnel blast and only have one artifact to sacrifice for the cost.

[Agrus Kos, Enforcer of Truth]

Chandra, pyromaster

−7: Exile the top ten cards of your library. Choose an instant or sorcery card exiled this way and copy it three times. You may cast the copies without paying their mana costs.

Do I have to cast all of the copies? Can I choose to only cast one? Say I choose a shrapnel blast and only have one artifact to sacrifice for the cost.

You don't have to cast all the copies. The ability says, "copy it three times. You may cast the copies without paying their mana costs." Therefore, you choose for each copy whether or not you will cast it.