Converting adjacency matrix into table with from/to/frequency

Question

I have a situation like this one:

id	a	b	c
a	0	0	0
b	0	0	3
c	0	11	2

The observations are frequencies of how many times a contacts b, ecc...

I need a dataframe like this one:

from	to	frequency
a	b
a	c
a	a
b	a
b	c
b	b
c	a
c	b
c	c