Let me give examples of firms from each of these market structures, though there really are not any firms that are truly in perfect competition.
The closest that we come to having firms in perfect competition is probably the farm sector. In the farm sector, there is really not much to differentiate one farmer’s produce from another. All wheat of a certain variety, grown in a certain place is essentially identical so there is no way to differentiate between firms. All eggs that are of a certain USDA grade are essentially identical. Firms that supply such products are as close as we come to perfect competition.
Monopolistic competition can be seen all around us. It is not a market structure for big firms with famous names because it is characterized by the presence of many small firms. The best example of this would be restaurants. While there are large chain restaurants, there are also many small, locally-owned restaurants that compete with one another and with the large chains.
Oligopoly is the market structure with the most recognizable firm names. In an oligopoly, there are only a few firms that control the majority of the market. The car industry is a good example of this. There are really very few major auto makers in the world and fewer in the United States. Another oligopoly would be the makers of operating systems for computers. There are only two major makers of such operating systems as practically all computers run on either Windows or on Apple’s OS. A third oligopoly would be airlines, since there are relatively few major airlines in the US.
Monopolies can be found in most American cities. Typically, there is one firm that enjoys a monopoly on cable TV services, a firm that has a monopoly for providing water, one that has a monopoly on electricity, and so on. Each of these firms has been granted a monopoly by the government and is the only firm that can provide that particular good or service.