Modified picture based on three different sources - citations needed?
It depends on what kind of information you are adapting from original pictures. 

If original picture has some very well accepted content in field which you are adapting, you can modify it and use it in your figure. In that case you don't need to give citations. For example, if you are using symbols used in original figures which are well accepted in field, you can use without citing original source. 

In contrast, if picture has some new content or some unique concept which you are adapting then you should acknowledge it. For example, if original figure is showing some unique symbol which is output of their new research or imagination, then you should acknowledge it. 

However it is always best practice to cite all images from which you have adapted content.