The 3% sounds nice, but if you're putting it into something conservative and leaving it there for three years, then it's just a 1% bump. Six years, it's only a 0.5% bump. Meh. I'd probably think of something else to do with that money, like maybe put it towards my mortgage if I still had one.
On the second question, I just made sure I put the next X years of expenses for my kids into conservative investments, then left the rest in long term investments. I think for me I chose X of 4, but it's really a personal risk / reward decision. And if you just make a decision and stick with it over the next four or five years, you'll probably end up just fine both practically in terms of getting their college paid for, and mentally in terms of having a plan in place and feeling comfortable with it.
I would encourage you to learn about the relatively new 529->Roth rollover option which generally allows you to roll unused 529 money into the kids' Roth IRAs tax free and penalty free. There are some rules and restrictions to know about, but they're not particularly hard to meet. If it works for you, it's likely better than distributing the leftovers and paying taxes and a 10% penalty on the growth.