Pixlr is a free web-based app that allows you to do basic editing online. Think of it as Photoshop's infant sibling.
You can do a lot of the same type of things you can do with Picmonkey and Canva, but Pixlr's the only one of the three that can free distort. Here are the steps I use to make a cover for a 3D book template.
First, open the website: https://pixlr.com/editor/, then click open image from computer.
Search for your 3D template from your files and open. This example is on a white background, but I recommend saving your book image on a transparent background.
Open a second file with the book cover you want to use. You have to first size it smaller by clicking on image and resize, then click the markee tool to copy and paste it onto your template image.
Click edit from the tool bar and then free distort in the drop down menu. Blue squares will show up on the four corners of your cover. Use your curser to drag the image, distort it, to fit the angles of your 3D image.
And voila. Click File then save. Be sure to save as a .png file for the transparent background.