AES-128 (cbc mode), is it possible to deduce the IV if the plaintext, ciphertext and key are known? I have the plain text, key and the ciphertext stored in DB. Can I deduce the IV based on this information?
By the formal modern definition of a cipher, the ciphertext and key should be all that's necessary to decipher, thus the IV of CBC should be in the ciphertext (often it is in the first block of ciphertext).

However the original description of CBC disagree and treat IV as a separate.

For the later situation, and assuming IV is not available (which suggests an exercise rather than a real situation), hint: write down the relation between IV, plaintext, ciphertext, and key by the definition of CBC; apply to the first block of plaintext; and use properties of bitwise exclusive-OR (commutativity, associativity, each element is its own inverse), and the defining property of a block cipher like AES-128 (that for fixed key, it is an easily reversible bijection of the block space).