"Can we aggregate the three accounts and call it a one-time rollover during the allowed 12 month period?"

If he submits them as Direct (trustee to trustee), it is not called a rollover and that rule does not apply. It's only when the taxpayer has access to the funds in between, giving them possible access to use the funds in the interim, that it falls under rollover rules.

"There should be no bank penalty for cashing in the IRAs since he is elderly."

You're not "cashing in" unless you intend to Lump Sum the money out of the IRA entirely? Now it is a taxable distribution and they no longer have that IRA account, no longer have any tax shelter of those funds, no longer are subject to RMD, etc. Is that what really is intended?

"into a single fixed account that pays 4.6% fixed rate"

Some types of investments should not be held inside of an IRA. Holding a Treasury security inside of an IRA removes the tax free aspect for State income tax, for instance. The reason not to do CDs is that the money is tied up for the length of the CD and CDs are still not earning much; not even like a high yield savings account or a Treasury note or bill.

Example: My one client that was recently widowed had a weird ladder of odd amounts of CDs from taking RMDs and using them to buy CDs. The highest CD was a 27-month and I think it paid .08% to July 2024 (that is not a typo). We lump sum distributed one IRA under $10,000 and one around $50,000 (inherited) because, at 86, she really should not need to worry about RMDs which were pretty high, they were a lot of the account balance, anyway, so just take it all, pay taxes on it, and move on. She forfeited the under $100 annually all these various CDs would have earned by cashing out most of them early. All these funds went into Treasury Direct and every investment there is over 4%. These are able to be sold on the secondary market, should you need the funds before maturity; otherwise, you can let it be used for repurchases. I helped her understand the Bond mutual funds she had held forever and she sold them and moved into some high yield low risk ETFs. She now has cash flowing and good growth.

"That is, if the bank is unwilling to write a check payable to "Vanguard FBO client's name","

Any institution holding an IRA account has to be willing to "send the funds" and they don't transfer by check, nowadays (electronic banking), when it is to another trustee. The Taxpayer would get a check, if they want to do an indirect (manual) Rollover and not do a Direct Transfer, or they want to do a lump sum distribution (or any distribution).

The destination institution should be able to help him with all of this. They are supposed to know the rules.