How should I cite 3 images that were used to compose a new image? I used three images from Wikipedia to compose them to a new image. All other images (not that many) are created by myself.

How should I cite them?

The images are:


https://commons.wikimedia.org/wiki/File:Computer-aj_aj_ashton_01.svg
https://commons.wikimedia.org/wiki/File:Server-multiple.svg
https://commons.wikimedia.org/wiki/File:Server_by_mimooh.svg


I currently use the following text below the description (within the caption):


  The image of a desktop computer on the top left is from
                \url{https://commons.wikimedia.org/wiki/File:Computer-aj_aj_ashton_01.svg}
                and was created by an unknown artist,
                the server image on the top right is from
                \url{https://commons.wikimedia.org/wiki/File:Server-multiple.svg} and
  was created by RRZEicons and
                the images that was used three times for classification workers is from
                \url{https://commons.wikimedia.org/wiki/File:Server_by_mimooh.svg} and
  was created by Mimooh.
The first image is sourced to OpenClipArt.org, which releases images into the public domain specifically to simplify your life in cases like this.  OpenClipArt specifically releases under the Creative Commons Zero license to enable this.  For such images, as well as anything else in the public domain (e.g., most images published by the US Government), there is no requirement for specific attribution.  

It is courteous to add a discrete note on the sourcing, and it would be dishonest for you to claim to be the artist, but the a major part of the reason that people build public domain image repositories is to let people build them into diagrams and other images without dealing with the attribution problem that you face. Since there are lots of these sorts of server diagram images on OpenClipArt.org, I would strongly suggest that you see if you can replace your current images with public domain images.

If you don't do this, then for the attribution-required images the figure caption should include the website references, just as you would reference a paper that you reproduced an image from.  For example, you may phrase it: "Server images adapted from [cite1,cite2]" where the citations give the appropriate online document citation, just as you would if you were citing any other webpage.  This is kind of a pain, and easy to lose track of if you reuse the images in other contexts, which is why I strongly recommend using non-attribution licensed images whenever possible.