Speaking at a London event on Thursday night, the 35-year-old admitted that he's worried he's going to be constantly exhausted with other little one to care for.
'Two is fine, I don't know how I'm going to cope with three, I'm going to be permanently tired,' Wills told a guest at the Centrepoint Awards.
He also joked that he'd really struggle if Kate Middleton, 36, gave birth to a duo of babies at the glitzy Kensington Palace ceremony.
'Twins? I think my mental health would be tested with twins,' Prince Harry's older brother quipped.
It's not the first time the 35-year-old has shared his honest views about parenting.
Attending a rugby match between Wales and Argentina in November last year, Wills made it clear than caring for Prince George and Princess Charlotte is a round-the-clock job.
William and Kate are welcoming their third child later this year
'I do need toothpicks to keep my eyes open because of the kids,' he said while meeting beneficiaries of a charity at the Principality Stadium in Cardiff.
The Duke and Duchess of Cambridge welcomed Prince George, who's now four years old, in July 2013, and two-year-old Princess Charlotte followed in May 2015.
The couple's newest bundle of joy is expected to arrive in April this year.
Wills and Kate have been very busy in the run-up to their third tot's birth - they've just returned from a four-day tour of Scandinavia.
The trip saw the duo, who tied the knot at Westminster Abbey in 2011, travel to Sweden and Norway for a number of official engagements.
While it's been reported that the pair have cleared their diaries to spend time with their brood during half term, they'll step out together on 18 February at the BAFTAs.