Fairly dividing a cake after some parts were burnt in the oven

There is a heterogeneous resource that contains both good parts and bad parts, for example, a cake with some parts burnt, a land-estate with some parts heavily taxed, or a chore with some parts fun to do. The resource has to be divided fairly among n agents with different preferences, each of whom has a personal value-density function on the resource. The value-density functions can accept any real value - positive, negative or zero. Each agent should receive a connected piece and no agent should envy another agent. We prove that such a division exists for 3 agents.