If he's RFA at the end then great.
If UFA then it's a bit much.
RFA VS UFA is overstated when players can and almost always do extend a year early.
The team loses a little bit of leverage by going straight to UFA, but they get some added cost certainty for that 3rd RFA year.
If the Sens went two years and both sides were far apart, the result would end up being the same, Pageau would go to arbitration and get a 1 year deal that would take him straight to free agency, except the Sens would be at the mercy of an arbitrator and would not have him cost controlled like his now. If the Sens aren't far apart in 2 years well it doesn't matter anyways because they can extend a year early and avoid unrestricted free agency.
If his salary was the same, his qualifying offer would be 3.3M since that is his year 2 salary. So that would be the LEAST the Sens could pay him for year 3 if he was on a 2 year deal.
There's really not as much benefit of keeping him as an RFA as people perceive in this situation. The real discussion point about this contract should be whether or not the Senators should have bit the bullet and locked him in long term. There's not much benefit to locking him in at 5+ years if the Senators think he has peaked, but if Pageau has another level and becomes a legit top 6 50 point two way C, he's going to have a significantly higher price tag after these three years are up than he might have had to lock in long term right now.
With how expensive his RFA years turned out to be, I think a 3 year deal was probably the right move, but it's hard to say without knowing what price was put on his UFA years. I'm not sure Pageau has that extra offensive gear. On the right team that is willing to use him in that role, maybe he could pump up his stats, but I'm not sure if he'll get that opportunity here.