I think Nintendo would have enough money, but as I recall Ubi is about twice the size of Ninty and they wouldn't want to grow the company that much, that fast. Plus, they don't seem especially keen on acquisitions so I doubt the entire idea would particularly appeal.
I don't think Phil Spencer could get approval for a couple of billion dollars for this kind of buyout, and Sony has worked hard to trim the fat in recent years, so I don't think they would want to add Ubi's 9000 employees to their payroll just after Kaz Hirai has worked so hard to turn the business around and make them profitable.
The other option I guess is one of the big three buying some shares in Ubi in order to allow the Guillemots + Nintendo (for example) to retain above 50% shares to ensure Vivendi cannot get control of the company. And then in exchange Ubi develops like 10 exclusives for that hardware manufacturer! Is that what 'white knighting' is? Again, I don't really see it happening as in theory none of the big three will lose much even if Vivendi take over, but it's more likely than an outright acquisition.