I'm doing all three (although I've only just started German). I would say that, personally, I find German hardest, but that might just be because I've studied French and Spanish in school when I was younger. I would say that I found Spanish easier because it's quite 'relaxed' in terms of grammar, it's pronounced phonetically and, like French, it shares a lot of vocabulary with English. But it really depends on which one interests you the most/your personal views. My brother can't stand French but finds German pretty simple.
Again, it really depends on which language you learn and how you learn it when it comes to having conversations.
I would say to familiarise yourself with the language or languages you want to learn you should start with using apps such as Duolingo and Memrise, just to get to grips with the basic vocabulary and pronunciation. Then progress onto watching videos and reading books in your target language: don't be afraid to look up words. And, as kaylee said, the best thing you can really do is to speak with natives!