I have read somewhere that Shiva and Brahma are manifestations of Vishnu himself and that he is the origin of all beings so I was just curious to know if his consort Goddess Lakshmi too is a manifestation of him.
No,Lakshmi is considered as a part of Goddess Adi Parashakti. Shakti's another name is 'Mahalaxmi' and Lakshmi's name is also Mahalaxmi and this is a proof that Shakti is Goddess Lakshmi. In Shakti'purana,Goddess Adi Parashakti said ''I am Brahma's wife Saraswati,Vishnu's wife Lakshmi and I am Shiva's wife Parvati''. This is also an another proof that Shakti is Lakshmi.