Ah, thanks! This is actually what happened, I opened an Affinity Designer -document my friend made so it was confusing since you could change the page size and all the "Include bleed"-options were there, but you couldn't change it in any way. I had never used the bleed feature of Affinity Photo before so I didn't know it was limited to stuff set in Designer.
It's not entirely true though that there's _no bleed_ in Affinity Photo, since there actually seems to be everything else EXCEPT the settings to change the amount of bleed. All the checkboxes to use these settings are there during the export.
And also for an image editing software I think it's completely ok to assume that the photos edited in it would go to print some day since all the color profiling is there as well. So I don't see why there wouldn't be bleed in a photo editing software (as there _almost_ seems to be, but not fully). The meaning of bleed feature (to me) is to make it automatic to export the 3mm on the sides instead of having to every time add my own crop markers.