解：过C作CF∥AB，∵AB∥DE，∴AB∥DE∥CF，∴∠1+∠ACF=180°，∠2+∠DCF=180°，∵∠1=120°，∠2=110°，∴∠ACF=60°，∠DCF=70°，∴∠3=180°-∠ACF-∠DCF，=180°-60°-70°=50°，答：∠3的度数是50°．
