“Two neighbors and I went together and put in a well for the purpose of irrigating our yards. We equally divided the cost of the well, pump, etc. Now the neighbor on whose property the well is located is selling her home. How can my other neighbor and I retain the legal right to use the well after the property is sold?”
What you want is not that you personally have the right to water from the well, but that whoever owns your property has that right (so that the water rights go with the property when you sell it). Therefore, what you need to do is have an easement drafted and recorded against all three properties which specifies not only that you have the right to use the well, but what the limits of that right are (e.g., you don’t want one neighbor draining the well dry and leaving none for anyone else) and what the responsibilities of each owner are with regard to maintenance, repair, and replacement.
A local real estate attorney should be able to help you draft something in proper legal form, covering all the necessary bases. If all the parties are in agreement, it should not be an expensive visit to the lawyer.