This adds to the base critical strike chance for your skeletons and skeletal mages. By default they have 5% chance to critically strike monsters. This affix will add 3% to that base so they will have 8% chance instead.
This is increased critical strike chance, which increases the base value. So if your skeletons have 5% base it would multiply the base by 1.5 so they will have 7.5% critical strike chance. If they have 8% it will increase to 12% etc. Hope it makes sense.
TLDR; When it says Added Critical Strike Chance, it adds to the base. When it says Increased Critical Strike Chance, it multiplies the base.