The pronunciation of the final S in plural words and verbs in the third person depend on the final consonant sound before that S.
The ending is pronounced /s/ after a voiceless sound, it is pronounced /z/ after a voiced sound and is pronounced /ɪz / or /əz/ after a sibilant sound:
- Voiceless: helps /ps/ -- sits /ts/ -- looks /ks/
- Voiced: crabs /bz/ -- words /dz/ -- gloves /vz/,
- Sibilant: buses /sɪz / or /səz /, bridges /dʒɪz / or /dʒəz /, wishes /shɪz / or /shəz /
Have a look on the examples at Woodward page