It isn't adding different amounts of Shock damage reduction to different characters. It is adding +3 Shock DR to characters who have different amounts of Shock DR from other sources (most likely just from their armor). In your case, without the boots, your barbarian has 9, Eder has none, Kana has 11, Durance has 5, Grieving Mother has 8 and Aloth has 8.
The confusion arises because armor lists DR as a general number, and then after it lists any exceptions (damage types that the armor has a different amount of DR against). So, for example, if your armor has a base DR of 5 and no exceptions it will only say "DR 5". If you then equip an item with +3 Shock DR your character sheet will then show "DR 5 Shock DR 8" - which will make it look like Shock DR just jumped by 8 unless you understand that you already had 5 Shock DR even though it didn't explicitly say it.
This is why I say that DR should just be listed explicitly for each type, rather than the current method of base + exceptions - the current method is easily misunderstood because it is not intuitive.
Nais, Thanks for explaining. So, additional DR just adds with your armor DR, seriously how could I be so stupid. Now everything clear. Cheers mate