Jakob Andreasen joins from Force India to take up the role of head of engineering operations to work under chief technical officer Pat Symonds.
Craig Wilson returns from Mercedes as head of vehicle dynamics, while Rod Nelson moves from Lotus to become chief test and support engineer.
Symonds said: "Williams is determined to make strong improvements in our competitiveness over the coming seasons and these new appointments continue our aggressive approach in recruiting some of the sport's best talent.
"We are also committed to allowing our existing talent to grow and showcase their skills in senior leadership roles. Our engineering team for the coming season is looking strong and I'm excited about what we can achieve moving forwards."
It has been reported that Ferrari's Rob Smedley could also be set to join Williams.