ok so ive done some testing, heres what i found
with lvl 170 items, and 34% in vitality to get max bonuses
with just 1 stat it gives 24 EP worth of that stat
with 2 stats it gives 14 EP to both stats, for a total of 28 EP
with 3 stats it gives 10 to all three, for a total of 30 EP
although…
with 3 stats, power gets 12 EP (4 power) worth of stats, and defense gets 15.33 EP (46 defense) worth of stats
and negative stats dont ‘count’ towards the 1/2/3 stat, so something like light mage pants will only fall under the 3 stat category despite having 4 stats (because one of them is a negative)