So basically, I've made a set of playing cards for a client and he said the printer wants to add 3mm bleed lines to the cards like .. However these don't look like bleed lines to me?(the pink line) Aren't bleeds on the outside of the image not inner? I'm guessing they're trim lines.. I've been looking for tutorials everywhere on how to add them with no luck.
This is a cut line, most likely for the print to get die-cut. You can create this with a stroke and then setting it to a spot colour called something like 'cutter'. The printer can then send this artwork to get a die made at the correct spec.
Your file should look more or less like this.
As @James C answered, it is a cut line. It should be on a separated layer. It does not matter the color of it.
But you also can send it on a different Pdf. I use the same name as the CMYK file and add My-PlayingCards-Cutline.pdf.
But your project should have trim lines. These will not be used to actually trim the paper, but to align the cut later on.
The "bleed lines" are the ones you use to define how much bleed you will add to your project, but they are only a reference for you. The printer does not need them. The trim lines are enough.
On this images I posted you can imply them.