Hanging multiple piece artwork need not be much more difficult than hanging single piece artwork.
Start with the middle piece and hang it as if it was on its own (See Hanging Artwork.) Then add 4cm to the width of an individual part and with a tape measure put a pencil mark to the left and right of the center.
Looking at the example below each part is 40cm wide so a mark is made 44cm to the left and right of the center nail. Using a level ensure that the two pencil marks are in line with the nail. If your artwork has more than 3 parts continue until you have enough pencil marks.
Hammer nails into the pencil marks and place the artwork on the nails. If you need to make slight adjustments you can hammer the nail up or down like in the photo below to make sure all parts are level.