swaplist3.py

import sys

print "Type a list of numbers with 3 in the middle."
myl = input('> ')
n = []
# Find the index of 3.
idx = myl.index(3)
try:
	n.append(myl.pop(idx))  # Removes '3' from the list and stores it in n
except AttributeError:
	print "Your list did not include brackets"
	sys.exit()
# Split the list at the index
foo = [myl[x:x + idx] for x in xrange(0, len(myl), idx)]

# Reverse the list of lists
foo.reverse()

# After the first list (at index 0), insert [3]
foo.insert(1, n)

# Flatten the list
bar = [item for sublist in foo for item in sublist]

# print the list
print bar

Note: does not work as expected when '3' is at index 1