In Excel, from the Menu Bar click:
Tools > Options...
Then click the View tab in the dialog box that opens.
Under Show make sure that "Windows in Taskbar" is checked, then click OK.
If the windows are maximized inside Excel, go up to the right hand corner of Excel beneath the Excel title bar and click the restore button. It is the one that looks like two squares.
Then right click the title bar for one of your windows inside Excel. Click Arrange... from the Shortcut menu that appears. Then select Cascade and click OK. That should make all of your windows easily visible inside Excel. You can try the other arrangements if you'd like.
Excel will normally open your files in an already open Excel window unless you open the Excel program a second time. If you have the Excel program opened multiple times it will slow your computer down because it treat it like another independent program.
If you like a certain way that a group of files opens, there is another method you can do. You can go to:
File > Save Workspace...
If you say have three files that you always open together, and you want them to open in the same configuration each time, you can save the workspace and when you reopen that workspace file it will open all of those files in that layout. One note of caution, saving the workspace does not save the individual files. So if you want to save the individual files make sure you click save to save them first.