How can I use a button to show an 2 images and a text box (3 items)?

You can put as many actions on a button as you'd like -- just use separate actions, instead of (or in addition to) the On Click tab in the button's Properties dialog box. (In fact, I usually don't use that tab and only use separate actions, so I can label them and see at a glance what that button does just by looking in the left-hand pane.) To add a separate action to the button, just select the button object and click the Add Action button (looks like a movie clap board) on the toolbar (or you can add the action from the menu bar, but that takes more clicks to do).

That said, your group idea should work. Just make sure you have your items in the right order -- items further down the left-hand pane are on top of those that come before (think of it as Lectora placing those items on the screen in that order). Then your button can just show the group.