@OP. coinmama is a legit crypto exchange allows people from many countries but never heard of oksdaq even google don't give much results for that name but few of them showing that is from South Korea or atleast the name on it but it seems like a complete scam and you lost that 3000 euros dude.
You are not wrong, Coinmama is a legit site that people can use to buy cryptocurrencies using credit card, but oksdaq.com is a scam site. If OP deposited money on that site, the money is as good as lost. It is very good good to look for legit sites to buy and trade cryptocurrencies, rather than the uncommon ones that can be scam. 
ICO,IEO,IDO and now DEFI, in future lot more to come so this is a lesson for him to stay away from hype trend investment schemes.