Apparently the youngest of the three was Grimbeorn, who is reported to have ruled in the Vales of Anduin in the year T.A. 3018. The evidence points strongly to his being born after Bilbo Baggins's visit to his father in III 2941, so his age could be no more than 77 at the time he is called 'The Old'. He may, of course, have lived longer that this.
Bëor was the first of Men to encounter the Elves, having led his people across the Blue Mountains into Beleriand. He was 49 at the time of that meeting, and died after another forty-four years of service to Finrod Felagund, at the age of 93.
The oldest of the three, though, was Aldor, the third King of Rohan. He lived for more than a century, 101 years to be precise, and ruled the land of the Rohirrim for no less than seventy-five years.