Here are the differences:
Dash: scam. Instamined, claims to be anonymous but isn't, spends buckets of money on marketing instead of letting the code speak.
Eth: Started out good. It's a turing complete cryptocontract platform, which basically means anything you can build in code (including other cryptocurrencies) can be build on ethereum. The problem is that development is very centralized, they are being guided by huge organizations instead of grassroots users. Price will probably keep going up for a while but I would actually put money into ETC instead of ETH because it's far more decentralized (in terms of development and direction) which means it stands a chance of becoming very very big (compare the "Internet" to "Microsoft Network").
BTC is losing market share right now as people look towards altcoins, it's typical gartner hype cycle situation - the prices will go up, then the vast majority of them will go to 0, but some will survive the culling and prove to be useful for some of the things they are claiming right now. BTC has already proven itself (in this regard), so when all these altcoins start heading to the floor BTC will be a safehaven.
That's my opinion anyway.
Eth development is least centralized. Many node implementations exist in various languages with a steog foundation and vitalik driving development.
Btc has steongest brand recognition and market cap and very stable proce appreciation reative to other coins in past 2 years.
Dash has unique and valuable consensus and governance mechanism.