I have just emailed a few publishers with my enquiry letter and 3 have come back to me to say they would like to read the full manuscript, is this a good sign?
Absolutely. Anytime you get a positive response from a legitimate publisher, that's great. However, before you commit to any company, get sound advice and do your research. Most publishers, not all, only accept manuscripts through agents.
Be encouraged, but always be on guard.