Three years from now Bitcoin will still be most secure cryptocoin, because it is most usefull coin and thus most valued. If some coin has 1/10 value of Bitcoin it is reasonable to assume the coin is 1/10 as secure
If Avalon didn't sell its mining rigs and chips to the public but instead got private venture capital and mined coins for Avalon itself, it would have easily been able to take over Bitcoin and all sha256 based cryptocurrecnies by sheer dominant hashing power.
Avalon didn't do it because, by pure luck, its founder happens to have the decentralization ideal. Avalon's 110nm technology is nothing cutting edge. I don't see why can't anyone fund a way cooler, hence more powerful, 40nm ASIC miner in a period of 6-12 months. BTC is a legal currency now. It's a viable business. If the new miners are 3 times more powerful than all Avalons, it can take over ~70% of the hashing power. The dominant role of BTC sure makes it a lightning rod to attract attackers.
If that happens or looks like going to happen, considering the difficulty to exchange BTC to a fiat currency, I see a huge drop of BTC price (hence more miners abundoning mining BTC) and a hugh number of BTC owners piling into alt cryptos.