Use a column value if not Null, or another column from a pandas DataFrame

# Import libraries
import pandas as pd
import numpy as np

# Create a sample DataFrame
df = pd.DataFrame(np.random.random(size=(10, 2)), columns=list('AB')).round(3)
df.iloc[[2,3,5,8], 0] = np.nan
df

# Create a third column, that will take column A value if not Null
# or fall back on B
df['C'] = df['A'].fillna(df['B'])
df

# Add some data
df['C'] = np.random.random(size=(10, 1)).round(3)
df.iloc[[1,4,5,8], 1] = np.nan

# Create a fourth column, that would take column C value 
# if the first two are Null
df['D'] = df['A'].fillna(df['B']).fillna(df['C'])
df