In your portfolio page, you would need to add another snippet call to each section with a custom template what outputs links to all of the pictures from the galleries. E.g. what is done in
this tutorial.
But the problem is, that you should not have same picture output twice on the html, because then slimbox will also show the picture two times. Usually this can be avoided by simply adding offset to the second call to tell it that it should skip the first certain amount of pictures. But this is not possible for you as you are displaying random pictures, so you don’t know what to skip.
So... actually, what you should do is to remove the &limit=`3` parameter from the call(s) in your portfolio page and create a custom template galleryPictureTpl where you use [+maxigallery.picpagenumber+] parameter in a PHx conditional to check if it’s over 3, and if it is, just output a hidden link to the image.. for the first 3 pictures, output the normal thumbnail code.
