The company’s challenges are a warning for others as the pandemic enters a third year. Peloton, the high-end exercise equipment maker, was one of the hottest stocks early in the coronavirus pandemic, gaining five times its value in 2020. But its struggles to hold onto all of those windfall gains — its shares are down more than 70 percent this year — show how challenging it is for stay-at-home stocks like Peloton and Zoom to adapt to the lifting of lockdowns, hybrid working setups and other habits taking shape in the third year of the pandemic.