You can shift funds from one beneficiary to another tax free. So if the first kid doesn't use up all the funds in their 529, you can shuffle those over to the second. This provides some flexibility in that if you're over the needed amount by a little you can stop saving for the next kid a bit early. If you manage to overshoot all three kids, you'd end up paying a 10% penalty and federal tax on the excess earnings. But, the excess is proportioned to be principal and earnings based on what was added and earned, so the penalty and tax doesn't apply to the whole excess. Also, if a beneficiary gets a scholarship, the 10% penalty doesn't apply to excess funds in that amount, though taxes still do, which will be higher than capital gains.
So a 529 isn't completely inflexible as long as you don't dump way too much into the account. If that can be done, no capital gains should outweigh any small amount of federal tax at your marginal rate on the excess. Since you have three kids, the beneficiary shift should mean no overages except on the third kid. By that point you should have a good idea about costs, and you might try to undershoot a little and pull the necessary remainder from after tax investments for the best of both worlds.
You can open a Vanguard 529 if you aren't avoiding state taxes, which means you'll get low fee investment choices. You aren't limited to your own state's plans.