AGNC Investment (AGNC -0.23%) is a real estate investment trust (REIT). However, it isn't a simple property-owning REIT -- it is a mortgage REIT. That changes the way investors should be thinking about AGNC Investment. A look at the last three years will help investors understand where AGNC Investment might end up over the next three years.What does AGNC Investment do?As noted, AGNC Investment is a mortgage REIT. It buys mortgages that have been pooled into bond-like securities, often using leverage in an a ...