Which is the least stable?

A line$-$angle formula shows a ring with six vertices. There is a double bond between the first and the second $($clockwise$)$ vertices. A CH$2$CH$3$ group is attached to the first and the second vertices. A CH$3$ group is attached to the third vertex.

A line$-$angle formula shows a ring with six vertices. There is a double bond between the first and the second $($clockwise$)$ vertices. A CH$2$CH$3$ group is attached to the second and the third vertices. A CH$3$ group is attached to the fourth vertex.

A line$-$angle formula shows a ring with six vertices. There is a double bond between the first and the second $($clockwise$)$ vertices. A CH$2$CH$3$ group is attached to the fourth and the fifth vertices. A CH$3$ group is attached to the sixth vertex.