It's the largest net lease REIT and has a lofty 5.6% yield, but you'll still need to watch this trio of issues.Realty Income (O 1.28%) is offering investors a 5.6% dividend yield today. That's well above both the market's 1.2% yield and the average real estate investment trust's (REIT's) yield of roughly 3.9%.If you're a long-term dividend investor, it probably makes sense to consider adding Realty Income to your portfolio, since it has increased its dividend annually for three decades and counting. But mak ...