What causes a hard rubbery movable bump under the tongue on the floor of the mouth behind the front teeth? It's been 3 months and doesn't grow.

Many possibilities. Could be cyst, partially blocked salivary duct, infection, benign or (rarely) malignant neoplasm. Any mouth lesion that lasts longer than 10-14 days deserves full examination by Dentist or Oral Surgeon.
Blocked Duct ? Just a guess. You'll need to see a local dentist to be examined. Rubbery bumps are not part of your normal anatomy so it needs to be evaluated.