First, check and see if there is public water. If so, then most likely the governmental authority for the area would require you to connect to their system. If not, then you will need to check with zoning and planning to see what are the requirements. Also, you will want to find out how deep are typical wells in the area. Installing a well can be a very expensive undertaking. If the current well is working well and able to support all three households, I would consider keeping the arrangement - but putting the terms down in writing and record the signed document.