The reviewer uses a spell checker AND dictionary to check spelling, and uses the AP Manual of Style or another standard to assure all the paragraphs, sentences, quotes and attributions follow proper style. The reviewer also checks the layout of each page to be sure headings and subheadings, colors, links and titles all follow stylistic consistency and function correctly. Adding a fourth member to a team is almost always helpful. The greater range of skills and viewpoints will prompt creative shifts to help keep a site dynamic. And having the back-up help is wise.
Additional people can help with various levels of review.
A new user can check for jargon and make the text more concise. A volunteer can check the flow through the site and verify external links. A young person or a senior who does a lot of browsing on the Web can suggest new styles, features, and methods. If these roles are new in your organization, it may be best to allow rotation of some of them, and to avoid specializing too soon. If the team members learn all the key roles, they can enjoy the variety and learning from one another while exploring their individual professional development.