The victims were murdered before the trip happened. The victims are neither Bob, Bill nor Fred, and aren't human either (so the victimes are now dead monkeys)
Since B,B and F were already there, the plotted a nice surprise for the Detective. Before going on the trip, they each prepared a body, so it will look similar to them and stored it in the trunk of the car.
When they arrived at the guesthouse and went to their room, they locked the door, snuk out of the window to fetch the similar looking dead body. Then they placed the body in the bedroom, locked the window, locked themselves into the bathroom and fell asleep.
What they didn't plan though was, that the sandwiches they ate were poisenous. Because B,B and F did quite some physical labour, the poison spread very fast through their bodies and they died later in sleep.
The detective survived the poisoning, since he ate less and after leaving the house in a state of shock (after knocking on the doors and not getting an answer) vomited into a bush around the house. Still dizzy from poisoning and in state of shock, the detective didn't realize that the bodys he found were actually monkeys.
edit:
Q: Does a dead human still qualify a human in this story?