from itertools import combinations

def swap(l, i, j):
	a, b = l[i], l[j]
	l[j] = a
	l[i] = b


def to_int(l):
	# Coverts list of digits to integer.
	n = 0
	for i, x in enumerate(reversed(l)): n += x * 10**i
	return n


def next_greatest(l):
	# Finds the next greatest number with the same set of digits.
	
	# First, the first index (from the right) where Left > Right digit.
	for i in range(len(l)-1, -1, -1):	
		if l[i] < l[i-1]: break

	if i == 0: return 0 	# No digits, found, smallest possible.

	# Find the largest number to the right of that index, including,
	# and swap. Then, sort the right descending.
	comp, largest = l[i-1], i
	for j in range(i, len(l)):
		if l[j] < comp and l[j] > l[largest]: largest = j

	swap(l, i-1, largest)
	l = l[:i] + sorted(l[i:], reverse=True)


def solution(l):
	l = sorted(l, reverse=True)
	found = False
	for i in range(len(l), 0, -1):
		# Get all combinations when removing digits.
		c = list(combinations(l, i))
		for m in c:
			m = list(m)
			while True:
				num = to_int(m)
				if num % 3 == 0: return num
				if not next_greatest(m): break

	return 0