These three terms are often considered interchangeable. IMHO, they're distinct. For example, you can live Green and not Living off the Grid (solar power to charge batteries to stream NetFlix). You can be Sustainable but not necessarily Green (use of chemicals to grow plants for your garden). Is there a concise definition of each which demonstrates their differences?
Sustainability = the ability to continue a particular behavior, lifestyle or process indefinitely
Living off the grid = living without using public supplies of utilities such as water, electricity or natural gas
Green = beneficial to the environment.
A problem with the above definitions is that many people will disagree and define these terms differently. As Ⴖuі also mentioned in his comment, especially sustainability is hard to define and there is no single agreed-upon definition.
I fully agree that the 3 terms are distinct. You can life off the grid and shoot endangered animals for food which would not be green or sustainable. Or you can plant trees in the desert which could be considered an attempt to be green, but is not sustainable1.
Personally I do think that green and sustainability have a lot of common ground. If you are not green and are damaging the environment a lot that's not going to be sustainable in the long run because ultimately we all live from what nature provides us. Living off the grid, or trying to be independent can be sustainable or green, but personally I believe that living off grid only makes being green and sustainable harder. There is a lot that could be improved in the society we are living in today, but isolating yourself from that is not going to help much. We are in this together, we should fix it together.