Stephens, competing for Great Britain, took third in the women’s 100m butterfly.
She clocked 59.94 seconds in the final, having earlier in the day swam 59.73 in the semis.
Stephens was only beaten by Russia’s Polina Egorova (59.00) and Hungary’s Petra Baroscasi (59.87).
The Plymouth College pupil had earlier won bronze in the 200m butterfly and 4x100m medley relay.
Fellow Leander swimmers Federico Poggio (Italy) and Kregor Zirk (Estonia) have also won medals at the championships.
Meanwhile, Tom Fannon will be in action in his favourite event on Sunday – the 50m freestyle.