If you haven't already, search for 'unicorn', 'third', 'NRE', and 'couple', 'couples' and so on. There are a number of threads on the forum from both couples seeking a third or fourth and the 'unicorns' they find (you would be the unicorn here). Many of the threads detail how things can go badly, badly wrong.
I am concerned about your possible situation because you will be dependent on this couple, even as you take over child care and household chores while you go to school. That can be disastrous for you. If there are problems, and there are always problems, you may not have the resources you need to leave or have the security to ask for changes, to ask for what you want and need in the relationship.
The rapidity of the relationship and how quickly both people in the couple want you to move in is suspicious. Many couples want a very long time before considering moving in a partner. Are they significantly older than you? You do not mention age ranges. Your youth is also worrisome - not in the sense that you are young and thus clueless - cluelessness is an 'age-free' condition. Young and dumb are not inevitably linked. But youth does mean inexperience. Here is where I am worried for you. Please, please read here and elsewhere about triads gone wrong, sometimes in outright predatory and abusive ways. Bluntly, and please excuse my crudeness, make sure you are not a nanny and maid they fuck instead of actually pay. I wish you all the best and I hope they are all you want.