If someone has been successful enough with grant support that they somehow manage to accumulate more than three months' summer support per year, what happens to the remainder of the funds above the three-month threshold?
Some people can buy off a course from their teaching duties in the non-summer months usually at the rate the department would have to pay to hire an adjunct to teach it. Some agencies will allow you to shift the salaries off yourself and onto a student so that maybe you go to 3 weeks from each of 4 grants rather 4 from each and use the saved money to fund some additional student time. This may or may not require permission from the funding agency, but it can usually be done without it as long as the PI isn't reduced to zero time.
With the NSF in the US, faculty aren't allowed to have more than 2 months of time funded across all NSF grants without explicit permission from NSF. Now, if you have 1 DoE grant covering 2 months and 1 NSF grant covering 2 months, they won't notice.
Some people also finagle this problem when their grants are staggered enough that there's only a short period (say one of three years) where this is an issue by pushing the money off to a future year and then exercising an option for a no-cost extension of one year to spend out the money.