The name of C4H8 with a double bond on the third carbon is 3-butene. Here's how we arrive at that name:

* C4H8: This formula indicates a four-carbon alkene (a hydrocarbon with a double bond).

* 3-butene:

* Butene indicates a four-carbon chain with a double bond.

* 3- specifies that the double bond is located between the third and fourth carbon atoms.

Let me know if you'd like to know more about naming organic compounds!