How do you mean?
1- Make the .tab elements all go the full width of the screen?
Us can try manipulating the width property and set it to 100%.
2- Make the three .tab elements take up a third of the screen each so that they collectively span the full width of the screen?
Try flexbox. Make their parent container a flex item and the use flex-grow on the .tab elements to distribute them evenly along the row. You can learn more about flexbox here https://css-tricks.com/snippets/css/a-guide-to-flexbox/
For a refresher on web basics, try http://learnlayout.com/
