Jaxx has a multi-currency option for IOS - its uses the Shapeshift.io exchange platform to trade between your currencies. I've not tried buying using this app so no comment on that - but I have utilised currency swaps and they work fine.
Also previously mentioned Coinbase offers the three main currencies BTC, ETH & LTC. However depending on your country it won't allow you to buy BTC from within the app.
I second a vote for Jaxx. Very slick and multiple currencies supported and shapeshift conversion. Also BCH support finally arrived.
Coinomi looks good but it is currently Android only, but seems to support all the hard forks at early stages.