Are these statements correct?

[kajtek]

1. Greg doesn't even consider changing his diet unless he is convinced by his doctor.
2. If the frost had hit later, this year's expected harvest of peach might be lesser.
3. Linguists at a recent congress in Seattle has warned that hald of human languages will have disappeared by the end of the century, as smaller societies are getting assimilated into national and global cultures. Are these statements correct?

[Alan]

Notwithstanding the typo (hald --> half in #3), all are grammatically possible.