Convert a repetitive list into Pandas Dataframe
If I understand this correctly your sequence is always 3 elements. Then you can do this:

a = ['A:1','B:2','C:3','A:5','B:5','C:6','A:2','B:5','C:7']
b = []
rep_len = 3

# Looping with step size equal to repetition length
for i in range(0,len(a),rep_len):

    # Selecting a repetition length
    c = a[i:i+rep_len]

    # Extracting everything in after letter and colon and casting to integer
    c = [int(x[2:]) for x in c]

    # Append to a list of lists
    b.append(c)

df = pd.DataFrame(b, columns=['A', 'B', 'C'])


Resulting in:

    A   B   C
0   1   2   3
1   5   5   6
2   2   5   7