What fuses for 100A, 3-ph disconnects
As @transistor already stated, I would give each blade/phase/leg a 100A fuse.

However: I have been working with mains power for decades. So while I would do that myself, I would never recommend that someone with little enough experience to need to ask this question (and I mean absolutely no offense here. Everyone has to learn some time, I'd just rather you not 'learn' by getting hurt/killed) be messing with 3-phase power unsupervised. Even when each phase is labeled 220V/240V, accidentally touching any 2 of them can hit you with ~340Vrms/~480Vp-p ... that's WHOLE LOT MORE voltage to push lethal current through you than accidentally touching 2 terminals on a 110V lightswitch!

So, yes you should get the 100A fuses (probably...depends on the wire guage you're feeding from the switch), but please, please have someone who is knowledgeable/experienced present to ensure that you can complete the work safely.