how to combine 3 lists into 1 list in pandas code example

Example 1: python merge two list

listone = [1,2,3]
listtwo = [4,5,6]

joinedlist = listone + listtwo

Example 2: append two list of number to one python

listone = [1,2,3]
listtwo = [4,5,6]
mergedlist = []
mergedlist.extend(listone)
mergedlist.extend(listtwo)