In this case, you trusted Coinmotion during the hardfork and as you don't have the private keys, you can't really do anything. You need to hope that they'll accept BCH and BTG in the future on their site.

And their site says:

The two options we plan to give our customers are as follows. When we are ready, each customer is presented with a choice.

We can convert your BCH to BTC automatically and add the amount to your BTC balance. We can send the BCH to your own BCH wallet address outside Coinmotion.

Our goal is to make BCH funds available in January 2018 at the latest.

https://coinmotion.com/blog/bcash-update/