Sum a Cubically cube's faces

Jelly, 16 14 13 bytes

3 bytes thanks to Erik the Outgolfer.

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Python 2, 155 150 147 123 121 120 bytes

Could probably be golfed quite a bit

Edit: -5 bytes by using a better method for removing the whitespaces

Edit: -3 bytes thanks to @Leaky Nun

Edit: -24 bytes by not removing whitespaces

Edit: -2 bytes by exploiting precedence

lambda n,a:[sum(sum(map(int,b[j*n:][:n]))for b in a.split("\n")[i*n:][:n])for i in range(3)for j in range(~i%2,i%2*2+2)]

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05AB1E, 16 bytes

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