This is not true: "The vehicle is 100% business use"

Because you also told us this: "that is used for all three of his business's"

The person is an Employee of the S Corps. That means this is Personal Vehicle used for the business of the Corporation, not His Business, for two of three entities.

"the use rate is about 10% (S-corp 1)"

He submits a Mileage report to his employer under the rules of An Accountable Plan and gets reimbursed at the Mileage rate. Not Percent Shared Usage. And that means, for the other entities, this missing share is Personal, not business.

"30% (S-Corp 2)"

He submits a Mileage report to his employer under the rules of An Accountable Plan and gets reimbursed at the Mileage rate. Not Percent Shared Usage. And that means, for the other entities, this missing share is Personal, not business.

"and 60% (LLC-rental schedule E filer)."

Which means 40% is Personal, from the perspective of the LLC.

"Client purchased truck with personal funds and title is in clients name, not corporation."

Employees turn in Mileage reports and get reimbursed under the terms of an Accountable Plan.

"I'm trying to figure the best way to allocate this vehicle"

Only the LLC gets to report the Vehicle.

The S Corps both have Mileage Expense paid to the Employee. They don't have a vehicle.

"Option 1- Keep vehicle in clients/LLC name and section 179 the asset (truck qualifies), basically LLC 100% use."

It's not 100% LLC use. You have a Mixed Use vehicle.

"Then have the two S-Corps reimburse the owner for actual vehicle use via accountable plan."

Reimburse the Employee.