Lets say that happens. You sign Glennon and you draft a QB at #3....aren't Glennon's days here immediately numbered? If not, doesn't that send a terrible signal about your organization? In your scenario, how are things supposed to play out? Glennon starts the first year, the #3 pick gets some time the 2nd year, and then what? Do you continue playing both guys? Do you actually conclude that Glennon is 'better' and look for ways to dump the #3 pick? To me, what you are proposing is the typical "Bears move" in that its the worst of both worlds....lets hire a GM, but force this new coach on him...lets franchise Alshon, but have no intention of signing him.
To me, if you are going to draft a QB #3, then Glennon's presence becomes a nuisance and is completely unnecessary. If you are going to sign Glennon to be your starter for the next couple years, then use that #3 pick on a guy like Myles Garrett or some other non-QB stud. I don't understand what you gain by signing Glennon and then drafting a QB.