sql sorting by subgroup sum data

How sort this

a 1 15
a 2 3
a 3 34
b 1 55
b 2 44
b 3 8

to (by third column sum):

b 1 55
b 2 44
b 3 8
a 1 15
a 2 3
a 3 34

since (55+44+8) > (15+3+34)