Letting A be one sock of color 1, B be one sock of color 2, and C be one sock of color 3, here is a list of the 54 possible combinations:
AABBCC
AABCBC
AABCCB
AACCBB
AACBCB
AACBBC
ABABCC
ABACBC
ABACCB
ACACBB
ACABCB
ACABBC
BAABCC
BAACBC
BAACCB
CAACBB
CAABCB
CAABBC
BBAACC
BBACAC
BBACCA
BBCCAA
BBCACA
BBCAAC
BABACC
BABCAC
BABCCA
BCBCAA
BCBACA
BCBAAC
ABBACC
ABBCAC
ABBCCA
CBBCAA
CBBACA
CBBAAC
CCAABB
CCABAB
CCABBA
CCBBAA
CCBABA
CCBAAB
CACABB
CACBAB
CACBBA
CBCBAA
CBCABA
CBCAAB
ACCABB
ACCBAB
ACCBBA
BCCBAA
BCCABA
BCCAAB