Using Imagemagick commands in Automator with passed images

I am trying to make a workflow in Automator that takes video screenshots and ideally at the end montages them into one image. I have the video shot working, and is saving into a timestamped folder, so the sequence runs and a folder with the current time stamp is saved 3 images Shot1.tiff, Shot2.tiff, Shot3.tiff into a folder. I've installed Imagemagick and it's working if I browse to the folder in Terminal and run the montage command: %> montage -geometry +4+4 Snap1.tiff Snap2.tiff Snap3.tiff montage.jpg then I get just what I want - the output montage.jpg However I'm trying to replicate this in Automator at the end of my 3 images - and that's what I'm struggling with - transferring this to a Shell script with the variables in Automator?