Boeing is flying high this year. Source: Boeing.

Investors have largely been disappointed with the stock market's performance so far in 2015, with the Dow Jones Industrials (^DJI 0.67%) opening the second quarter with a loss yesterday and sinking further into negative territory for the year. Yet as much as investors want to see the Dow keep rising to record gains, they should appreciate that without the efforts of just a few stocks, the index's losses could have been much worse during the first quarter. Specifically, UnitedHealth Group (UNH -1.98%), Boeing (BA 0.39%), and Disney (DIS -0.55%) have all posted double-digit percentage gains since the beginning of the year, and their share prices are high enough to have had a sizable impact on the price-weighted Dow as a whole.

Source: UnitedHealth.

For UnitedHealth, numerous moves have helped push the stock up almost 17% this year. The most recent was the health insurer's acquisition of pharmacy benefits manager Catamaran (CTRX.DL), which will combine with UnitedHealth's OptumRx to create a much larger player in the PBM space. The health insurer has also more aggressively participated in the healthcare insurance exchanges under Obamacare this year, with many of the kinks in the system having been worked out in its first year. As it grows and gains more market power in healthcare, UnitedHealth has the capacity to help its customers save more money while keeping its own profit margin wide, and that could lead to further gains throughout the year.

Boeing, meanwhile, is up 15% largely on the same trends that have helped it climb precipitously over the past several years. Even with the plunge in crude oil prices and their consequent impact on fuel costs, airlines don't appear to have slowed their drive to replace older airplanes with newer, more energy-efficient models from the aerospace giant. As the company has addressed production concerns, backlogs have remained at high levels, giving Boeing plenty of years' worth of business to catch up on before it has to worry about generating a single new aircraft sale. At the same time, Boeing is using creative strategies to structure new orders to reflect changing demand, trying to build in flexibility to drive orders to aircraft models that need greater volume in order to free up capacity for the most popular airplanes.

Source: Disney.

Finally, Disney has gained 12% as the multimedia entertainment giant keeps firing on all cylinders. With plans for a new Star Wars installment, a sequel to the hit animated feature Frozen, and its usual host of theme park, studio and television content, and retail success, Disney has translated all of its opportunities into huge profit gains. Without any obvious limits to its immediate growth, Disney can afford to direct its attention to its best opportunities while keeping a huge backlog of other ideas in reserve for later use.

Without the big lift these three stocks have provided, the Dow would have fallen far more during the first quarter than it did. As long as these companies remain strong, moreover, the stock market should continue to see only moderate pullbacks and could keep putting off the inevitable correction that so many anticipate.