All three of the professions employed people who were not likely to become rich. All three professions also depended on railroads to move their goods to Eastern markets. All three jobs were relatively dangerous. While the three professions started as relatively small professions, ultimately all three of them were dominated by corporations and other large producers who controlled much of the land and resources.

The mining frontier refers to miners removing minerals from the West. American mining efforts in the West started at Sutter's Mill and continued through the Klondike Gold Rush in the late 1800s. Early miners created boom towns that were relatively lawless and filled with transients who went from town to town looking for the next strike. Whenever a deposit was fully mined, the miners often moved on, leaving ghost towns. Later, mining corporations would come and use the old mines as well as dynamite and chemicals to remove more ore from the ground.

The cattle frontier took advantage of the free-range cattle of Texas who had gotten out and reproduced during the Civil War. Cowboys, young men looking for adventure and a potential big payday, supplied the labor needed to get cattle from Texas to rail hubs in Kansas where they could be transported East to better markets. Many cowboys died along the trail due to such hazards as stampedes. The cowboys drove the economies of rail towns since the young men spent freely on various goods and vices. The open range was closed off by homesteaders who controlled the water in the region. Over time, corporations bought the cattle and homesteads in order to create their own potential beef monopolies.

The farming frontier referred to the sodbusters who came to the Western Plains thanks to the Homestead Act of 1862. These farmers were at the mercy of railroads, who raised rates on goods being shipped East. Many farmers also lost their money due to crop failures and falling commodity prices. Corporations also bought up the small farms to create large-scale factory farms where the original farmers were treated as employees instead of land owners.

Historians of westward expansion have on occasion divided up the settlement of the American West into various frontiers. These were often in reference to the competing needs of the settlers. As your question implies, there were three main groups of settlers in the West: farmers, cowboys, and miners. Let's look at some ways they settled different, but overlapping, frontiers.

By the mid-19th century, ranchers (aka cowboys) kept large herds of cattle in the territories of the American West. There were vast grasslands for their herds to use for grazing. For them, the frontier seemed endless. Most of the land that they used as pasture was owned by the federal government. This land provided them with a large degree of autonomy.

This changed in 1862 with the passage of the Homestead Act. By this time, the US Government wanted to encourage farmers to settle the region. Farmers were more likely to establish permanent communities and develop the land, something that cowboys were not interested in doing. As a result of the Homestead Act, numerous settlers moved into the region with the hope of establishing new communities and pushing back the frontier. As farmers began cordoning land off for growing crops, they often came into direct conflict with the cowboys who wanted to keep the land open as pasture.

Throughout all of this, mining corporations and some individuals were moving operations into the frontier in search of valuable minerals. The California Gold Rush was one of the most important events in the settling of the western frontier. Mining also resulted in the first major push of American settlers into the Rocky Mountains. Throughout the second half of the 19th Century, miners were often dependent on the expansion of the railroad in order to move equipment in and minerals out. Because of mining operations, many mining towns were established deep in the frontier close to where mineral deposits were found.

The statement to which this question refers emphasizes the economic diversity of the American West. One might add many other economic pursuits as well—logging, railroads, and trapping in particular. One point of comparison between mining and ranching is that, while they are popularly remembered in terms of rugged individuals like the "49er" and the "cowboy," they were in fact dominated by large corporations. Cowboys and miners were mostly wage laborers. Farmers, too, experienced the effects of economic consolidation, though they mostly owned their own lands. They quickly found themselves in debt to banks and struggling under the weight of national monetary policy and extortionate railroad shipping rates.

Indeed, the "cowboy" and the "miner" frontiers were also dependent on the railroads, which were the key to American expansion. Another commonality between these economic "frontiers" is that they were a product of immigrant labor, including Irish, Chinese, and Mexican laborers. In short, it makes historical sense to think of the West not as a monolith, but as a collection of diverse economic pursuits and experiences.