Why BH3 its classified as a "Molecular Hydride" and AlH3 its a "Intermediary Hydride"
Alane is not a molecule: https://en.wikipedia.org/wiki/Aluminium_hydride You can't isolate molecules easily because they polymerize.

Borane on the other hand is known to exist as a dimer: https://en.wikipedia.org/wiki/Diborane

The difference in bonding is likely due to "hypervalency" (or similar concept with different terminology) in aluminum.