So, this is something I'm a bit curious about and can't really seem to find any clear cut answer to in my examination of the rules for magic items. I'll try to be vague just in case players stumble in on this thread.
Suishen is listed as having a certain number of 'powers'. Most of those powers are listed as a 3/day use. This indication is essentially identical to the way sorcerers/oracle/bards/monsters list their spells as well, and I've always just taken it to mean the powers are used just like that, 3 spells per day from said list.
Though when Suishen powers up, I noticed some strange wording that has me questioning myself if I've misinterpreted it entirely. One of the powers that is listed as a 3/day changes to an 'at will'. The fact that it listed that specific power as three times per-day prior to the upgrade has me doubting my interpretation of how it was supposed to work. Can the sword 'cast' each of its listed powers 3/day? Or does it cast 3 spells per day from said list?
Yep, that's pretty much it. Spell like abilities and magic item powers have a number of uses per day as listed for each one.
The X/day as a pool only works for spontaneous casters, and only for their spells. Things such as bloodline powers or a Paladin's lay on hands ability also have individual limits.