The Way I have seen this working is that if, for example you have an item that gives +60 mit vs crushing, and not to any off the other two, the % value in the crush subsection will increase, however so will your total mitigation, your overall mitigation seems to increase by a third of the individuals value, and thus when an item has for example +140 to each type of damage (eg empowered greaves) it gives the overall mitigation 140 extra as well as the individual types in the subsection extra.
As to maxing mitigation out, unlike someone else mention, the cap isnt 5600, but 6200, so when at 6200 you have 80% mitigation against a level 70 component. however as far as i am aware about the individual slashing, crushing and piercing section in the mitigation these will cap at 8% and i believe its in addition to the 80% base mitigation.
Overall due to the items + value to one of the types, giving an overall value as well, some items will be worth staying with, however raw health will always be a good choice.
Personnally i use a mixture of the two. I use breath of netshiva (the feerrot spire dragon) +10 to all stats +60 to crush/slash, as well as dark chitin ring and light chitin ring, also using a lot of the dragonscale items from the claymore line, using the earing for raw hp and the mitigaion although no stats, but the bracelet with no raw hp for the +30 to each stat. I think really, just switch items around and see which you like more.