A length-extension attack works by taking the output of a hash and using it as the starting point for a new hash; you can do this even for hashes of messages you haven't seen, minting new related hashes. Truncated SHA-2 hashes don't output the whole hash, and so you aren't given enough information to start a new related hash.

Because they're already truncated.

SHA-384 is SHA-512 with a different IV (which doesn't affect LEAs) truncated to 384 bits (which gives you 128 bits of resistance against LEAs).

SHA-224 is the same story but with SHA-256 instead (and only 32 bits of LEA resistance).