Final answer:
The three key assumptions about the long-run dynamics in a perfectly competitive market include the attraction of new firms by positive economic profits, the exit of firms due to losses, and the achievement of long-run equilibrium with zero economic profits.
Step-by-step explanation:
The three assumptions about the long run in perfect competition are as follows: First, in the long run, positive economic profits will attract new firms into the market, leading to increased competition. Second, economic losses will cause existing firms to exit the market. Third, a long-run equilibrium is attained when no new firms want to enter and existing firms do not want to leave the market, as economic profits have been driven down to zero. This is due to the fact that firms respond to profits by increasing production and to losses by reducing production or exiting the market. Ultimately, this leads to a situation where all firms earn zero economic profits, producing output levels where price (P) equals marginal revenue (MR), marginal cost (MC), and average cost (AC), indicating no incentive to adjust the market status.