He's already 3+ times older than she is. There's no need to wait any time to be at least 3 times older. He already is.

IF he waits 12 years, he'll be 120 and she'll be 40. 3 times 40 = 120

but according to actuarial tables he won't live more than 2 years. at age 108 your expected life span is very short. Odd are good he'll never reach an age where he's exactly 3 times as old as she is.