You have to contact poloniex to resolve this issue if you believe 3.5BTC really have been taken by them.
Overall poloniex is a good platform with nice trade volumes, that's exactly what we need now for Execoin!

So please vote and leave comments on their website: http://poloniex.freshdesk.com/support/discussions/topics/1000011017, it's critical for Execoin to be added to other exchanges as soon as possible!