When talking about e-commerce, investors often focus on Amazon.com (NASDAQ:AMZN), the 800-pound gorilla of the market, and the industries it's disrupting with its ever-expanding ecosystem.

However, they shouldn't ignore three other high-growth companies that are changing how companies do business over the internet: Shopify (NYSE:SHOP), Square (NYSE:SQ), and GrubHub (NYSE:GRUB). All three stocks have generated multibagger returns over the past three years, and they could soar even higher on some long-term tailwinds.

Dollar signs rising out of a smartphone.

Image source: Getty Images.

1. Shopify

Shopify assists companies in designing, setting up, and running online stores. It helps them process orders, payments, and shipments; launch marketing campaigns; build online customer relationships; and measure their overall performance with cloud-based analytics. Simply put, it's a one-stop shop for building a company's e-commerce presence.

New features like payments platform Shopify Pay, a point-of-sale (POS) card reader, a wholesale channel for buyers, bulk label printing options, and APIs (application programming interfaces) for integrating its platform into third-party apps have expanded its ecosystem. Shopify also offers merchants cash advances through its financial arm, Shopify Capital.

Revenue rose 62% year over year last quarter, as subscription solutions and merchant solutions revenue soared 55% and 68%, respectively. Shopify expects its full-year sales to grow 51% to 53%, compared to 73% growth in 2017. The company isn't profitable on a GAAP basis, but it's been profitable on a non-GAAP basis for the past four quarters.

2. Square

Square disrupted the market for traditional POS systems with the Square Reader card-reading dongle for smartphones, the Square Stand POS solution for iPads, and the stand-alone Square Register. It then tethered its merchants to an expanding ecosystem of cloud-based services for managing payroll, inventory, customer relationships, deliveries, shopping data, and website designs. It even loans money to merchants through its financial unit, Square Capital.

The company also owns the food delivery app Caviar, which it recently integrated into its unified Square for Restaurants bundle of services for booking tables and managing payments. For consumers, it offers Square Cash, one of the most popular mobile P2P (peer-to-peer) payment apps in the U.S. Square Cash is also the only major P2P payments player that lets its users buy and sell bitcoin.

The Square Register.

Image source: Square.

Square's revenue surged 48% annually last quarter. On an adjusted basis, which excludes transaction-based costs, bitcoin costs, and deferred revenue adjustments, revenue climbed 60%. Square's total transaction-based revenue and gross payment volumes both grew 30%. Square still isn't profitable on a GAAP basis, but its non-GAAP earnings surged 86% last quarter. Looking ahead, it expects its adjusted revenue to jump 54%-57% for the year and for its adjusted earnings to go up 56%-70%.

3. GrubHub

GrubHub owns the top online food delivery platform in America. Its portfolio of services -- which include its namesake app, Seamless, Eat24, MenuPages, Allmenus, DiningIn, and other apps -- controls about half of the U.S. food delivery market, according to Second Measure's March estimates. Its closest rivals, Uber Eats and DoorDash, controlled 21% and 15% of the market, respectively.

Not only does GrubHub continually scale up by acquiring smaller rivals, but it's also building an ecosystem atop its high-growth niche. The company recently acquired LevelUp, a payments and loyalty services provider, which could help it counter Square for Restaurants and cross-sell more services to its growing list of restaurant partners.

Revenue surged 51% last quarter, thanks to GrubHub's acquisition of Eat24, 35% growth in daily average grubs (orders), 39% growth in gross food sales, and a 70% jump in active diners.

GrubHub's mobile app.

Image source: GrubHub.

Unlike Shopify and Square, GrubHub is consistently profitable by both GAAP and non-GAAP measures. Last quarter, its non-GAAP net income rose 99% year over year, GAAP earnings more than doubled, and adjusted EBITDA grew 61%. Wall Street expects GrubHub's revenue and non-GAAP earnings to grow 44% and 59%, respectively, this year.

High risks and high rewards

Shopify, Square, and GrubHub generate a lot of growth, but all three stocks have lofty valuations. Shopify trades at over 230 times forward (2019) earnings, Square at almost 100 times forward earnings, and GrubHub trades for nearly 60 times forward earnings. Those valuations are based on non-GAAP figures -- they would be even higher if GAAP figures were used.

Therefore, these stocks definitely shouldn't be core holdings for most investors. However, investors looking for riskier investment opportunities with potentially high returns should take a closer look at these high-growth digital retail plays.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Leo Sun owns shares of Amazon, Grubhub, and Square. The Motley Fool owns shares of and recommends Amazon, Shopify, and Square. The Motley Fool has the following options: short September 2018 $80 calls on Square. The Motley Fool has a disclosure policy.