Not much harder than creating a collision in each of the hash functions, sequentially. You can't combine two broken hashes into a strong hash. But as long as at least one of them remains unbroken, then their concatenation is clearly secure too - assuming you keep the full result of each hash function and concatenate them into a long output, rather than trying to combine them somehow. Since it's impossible to know how resilient a given hash function will turn out to be against future cryptanalysis, it can make sense to hedge your bets by combining dissimilar functions.