I need to chart a complex dice system probabilities: how to compute them?

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Short intro for readers that plays old boardgames: we're talking about Silent Death dice system.

Long intro: I roll 3 dice, each of them can be a 4,6,8,10 or 12 faces die. Lets call H the sum of them all. There's 3 other values i can calculate: (i'm trying to define them with my poor english, pls be patient :) )

  • A: the sum of the lowest values rolled
  • B: the sum of the middle values rolled
  • C: the sum of the higher values rolled

Examples:

roll is 2, 5, 9
 - A = 2
 - B = 5
 - C = 9
 - H = 16

roll is 5, 5, 9
 - A = 10 ('cause there's 2 "lowest" values (5) )
 - B = 10 ('cause there's 2 "middle" values (5) )
 - C = 9
 - H = 19

roll is 2, 5, 5
 - A = 2 
 - B = 10 ('cause there's 2 "middle" values (5) )
 - C = 10 ('cause there's 2 "highest" values (5) )
 - H = 12

roll is 5, 5, 5
 - A = 15
 - B = 15
 - C = 15
 - H = 15

I need to build a spreadsheet to chart A, B, C and H chances given the 3 dice faces.

How do I calculate that probabilities?

(If you can write the answer also in excel formula, other than in math expressions, it will be really apreciated :)

-edit-

This just 'cause i understand more that format than math symbols, not to make you do my work )