How do I determine shaded cells in K-tables for minimizing multiple outputs?

[Korisnik]

Homework Statement

http://i.imgur.com/VYPECuW.png?1 (f1: f2: f3: f1*f2: f1*f3: f2*f3: f1*f2*f3: is what's written near tables)
I need to minimize the functions (sum of minterms: f1 0, 1, 2, 4, 5, 11, 15; f2 0, 2, 4, 13, 15; f3 0, 1, 3, 4, 5, 7, 13, 15) using K-tables... This is what they show as procedure (they write after the picture:
f1 = c+d+f
f2 = b+c+e
f3 = g+b/d + e/h
and so on...

I see those (c, d, f, b, e, g, h) are the (essential) primary implicants in which there are shaded cells. I cannot determine how to find these shaded cells! I can't find the rule they used to determine them... I see that they must be in essential prim. implicants, but if I added another rule: for example they must not be in the f1*f2*f3, then why isn't also minterm[2] in f2 shaded? I've found some tutorials on the internet but they don't use these steps...

Thx in advance!
(just to make it clear, i don't need a full solution, only how to find these shaded cells ~ what do they mean)

 

[Korisnik]

Could someone help? I know how to do it using Quine McCluskey, I just can't understand what happened in that step; what's the method of solving these using k maps. Links, tutorial, anything?

 

[Korisnik]

Is the question unclear? Does nobody know how to minimize this?