Midfielder Agg joined in January from Brighton & Hove Albion, while 18-year-old centre-back Katie Jones made her debut in 2015.
Striker Sawyer, also 18, played her first game for the Vixens against QPR in the Women's FA Cup.
Midfielder Jodie Brett has returned to parent club Chelsea after a loan spell last season.
Lily Agg, Katie Jones and Paige Sawyer have all left Bristol City ahead of the new Women's Super League One season.