i’m newbie till now i use 3 sites and recommended this bittrex.com

I personally use bittrex; I've used it for more than half a year and I had no problems whatsoever. I'm not sure, but I think Poloniex is still having complaints in terms of customer support, as I've read a good amount of Reddit posts months ago about their funds getting locked up for months. I personally wouldn't risk it.

Why not just try all 3 out and use what you personally prefer most?

Of those three Bittrex has the best reputation. I personally like Poloniex as well, but their support appears to be lacking, especially with alt coin withdrawals / deposits.

Of Yobit I'd stay clear. Their investbox and non-provably-fair gambling make them a bit shady.