Answer: 2 hours.
The small rabbit can dig a warren in nine hours so digs 1/9 in one hour.
The medium rabbit can dig a warren in 6 hours so digs 1/6 in one hour.
The large rabbit can dig a warren in four and a half hours so digs 1/4.5 in one hour.
1/9 + 1/6 + 1/4.5 = 0.5. So in one hour, they have dug half a warren between them, the whole warren will therefore take 2 hours. QED.