I'm amazed that Microsoft still include Paint with Windows. It wasn't much use when then brought it out over 10 years ago and, with advances in software design, it looks even more pathetic now. However, since it's all you've got at the moment, I'll try to tell you how to do what you want to achieve:
Open Paint.
Select File > New (This will give you a blank white 'canvas' to work on).
Select Edit > Paste from.
Navigate to the location of your first picture and double-click on it. It will now appear on top of your white canvas. (It will probably be bigger than the entire canvas so, initially, you'll no longer be able to see the white background).
[NB: If you don't see the picture file in the folder where you expect it to be, check that the 'Files type' box shows 'jpeg files' or 'all files'. Paint defaults to looking for bitmap files].
Adjust the size of the picture by placing the mouse over a corner of it, so that the cursor turns to a diagonal arrow. Click and drag the corner to change the size. (Only drag on corners. If you drag on the sides you won't retain the aspect ratio of the picture).
Adjust the position of the picture by placing the mouse within the picture, then click & drag.
When you're satisfied that your first picture is correctly positioned on the canvas, select Edit > Paste from. Repeat the previous procedures with your second picture. Then use the same process to add the third picture to the canvas.
When you've finished, go to Edit > Save as. (Choose a name for your new file and check that the file type is shown as jpeg).
If you want some decent photo manipulation software on your PC, get Serif PhotoPlus 6.0 (free of charge) here:
http://www.freeserifsoftware.com/
Chris