Are you wanting to assemble 3 images as a "collage" or are you wanting to stitch 3 images into a panorama?
Most image editing apps, beyond the most basic, can do the former, the "collage" assembly. Those listed by 960design all all decent choices as are many others.
For the latter, you need either one of the more powerful general purpose tools that include a stitching function (e.g Photoshop, Lightroom 6/CC, ...) or a special purpose stitching tool (e.g. Hugin, ...).
i m giving you the information of best image online editor. i think online image editor is best then installed Photoshop. pixler is awesome site for online editor. i use this site personally