I would never start a company without a buyout/vesting agreement (*). I admit when I first saw it I shared the same concerns of your cofounder, but it's necessary to avoid somebody being able to walk away with a significant part of your company. Things happens, people change their mind, and it's not an uncommon situation to lose one or more cofounders. The company must be able to survive in these kind of situations.
So for me, if somebody refuses the buyout agreement, he cannot be part of the company.
Similarly, "unanimous consent on all decisions" it's absolutely a no-no. A startup needs to move fast, and you don't want to waste days or weeks until you have unanimous consent. At the end of the day, the CEO is the person who will make the call; of course decisions can be delegated, and it's the CEO's job to make sure everybody is happy with the company strategy, but one person needs to be able to decide when things get complicated.
Exceptionally, a few decisions can be subject to board of directors approval; one of those might be the removal and buyout of one of the founders. In this case, assuming the BoD is composed by the three founders, the majority of the BoD should approve this decision.
At the end of the day, if this cofounder "perceives the company as 1/3 vs 2/3" I don't really understand why he's willing to be part of the company.
Edit: (*) The buyout agreement must be time-limited. For instance, the company can buy back 100% of the cofounder shares before he spent a year in the company. 66% after a year and before he spent two years, and so on...