Few years ago I've read about this question the results from a study upon. They were saying that it was easier to learn several languages than once. And now, I can confirm to you about cause I've compared both methods. In effect, I think it's merely better cause you are in a logical way for each language who allows you to no blend and, at the opposite, to divide correctly all words in order to the languages's customs. It's nice, and it's more easy yet when you find some one for speak on Skype for instance, that makes you improve in an obvious manner too :)

Sometimes I get my German, French and Italian vocab mixed up.

I found this advice: http://how-to-learn-any-language.com/e/faq/problems-several-languages.html

It's never easy and absolutely confusing learning several languages at the same time, but it's also very exciting (especially when you have to memorize all those "weird" letters). ^_^

I think its great! :) Its really interesting to learn and compare the differences (and similarities) between the different languages, especially learning the same concepts in all the languages, at the same time. But to answer your question, I don't find it any more difficult than learning one by itself! You get out what you put in :)