title_body: I am looking for a practical example of a smart contract with three parties
upvoted_answer: Your example is interesting and could be really meaningful if real world data (e.g. weather data) can be included.
  Here's a simpler example that I can think of based on your example.
  Let's say John put up the fence as Bob instructed, but there is misalignment between Bob's instructions and Alice's requirements. Now Alice is thinking about suing Bob, because in her view, Bob acted against her will. But Alice also does not like the courts. But she heard about this dispute resolution DApp on Cardano (to be build) where an independent third party helps the disputants solve their dispute.
  This third party can be a mediator that simply helps the parties find a mutually agreeable solution. The payment of said mediator can be in the form of a smart contract. Alice opens a case and initiates the payment of the mediator for her share (usually mediators and arbitrators get paid an equal amount by both parties). This amount is now in escrow and will only be dispersed if the other party (Bob) agrees to mediate this case as well (by paying the mediation fee).
  Instead of having two contracts (one between the mediator and Alice, the other one between the mediator and Bob), both transfers can be rolled up into one smart contract.
  Of course, there are many more interesting examples, but I hope this example helps. Btw, I will be proposing a dispute resolution marketplace in an upcoming Project Catalyst fund, so keep an eye out for that. There's a lot of little disagreements between neighbors (as you indicated) where smart contracts has the potential of making life a little bit nicer for communities of varying shapes.