"Also, should I open all three types of accounts for them and place 1/3 of the funds in each account?"
Your child can not have a Roth IRA in their name unless they have compensation earned from working.
With a qualified tuition plan, you can withdraw the earnings tax free if used for higher education. You may get a state income tax deduction on contributions although you won't get a tax deduction from the federal government on contributions.
With a brokerage account, you need to start by learning the difference between putting the funds in the child's name (a bad idea) or a custodial account (better), and you need to understand that annual earnings will be taxed at your income tax rate, this is called the "kiddie tax" and it prevents you from putting money in a child's name and paying lower income tax rates because they have less total income than their parents.
https://www.fidelity.com/learning-center/personal-finance/custodial-account-for-kids
https://www.irs.gov/taxtopics/tc553
With a brokerage account, the child is guaranteed to pay income tax when they withdraw the money, at capital gains tax rates which are currently lower than ordinary income (but might not stay that way). With a QTP, qualified withdrawals are tax-free, while non-qualified withdrawals will be taxed as ordinary income (higher than capital gains rates) and may be subject to an additional penalty.
Whether to open a custodial investment account or a QTP depends on how much flexibility you want 18 years from now, and whether you want to pay some tax guaranteed, or maybe no tax or maybe higher taxes. You could also hedge your bets and do both.
But no IRA.
