When paired with a Fenrir friend you already have 3 jammer creation actives to use. Since Fenrir is on a 5 turn cooldown and gives a 1 turn skyfall then most of the time those 3 is all you need. However I could see having another Fenrir being nice to have in a few situations. TL;DR: For almost all cases, you only need to own two dark Fenrir. However 3 could be seen as a luxury for certain dungeons or setups. Also since Fenrir has a low cooldown and very good ATK stats and awakenings, you could also SI something else onto him and put him in another dark/devil team for damage output.