Each disk that is bootable should have an .iso file that can be extracted then written to flash stick using UltraISO trial version.
On File tab Open DVD/CD Image, then on Bootable tab Write Disk Image.
You will have to study the help file or research further how to merge the 3 ISO's if they are all image disks and one not a drivers
/apps disk, or try Deacon's approach to write the first disk's image to stick and place the files from the two others on the stick to see if it will proceed. If not you can format other sticks with subsequent disks, or write each one as you go.