Sure, why not. Kinda irrelevant though, given that you need a Big 8 to win the 'chip today.
I voted 'no' but your post reminds me of Surferdad's 1-3-8 rule-of-thumb to creating a championship roster:
1 - You need one truly elite superstar, sure-fire HOFer.
3 - You need 3 all-stars
8 - You need 8 players who could be starters on most NBA teams.
The model is the 1986 Boston Celtics with Bird-McHale-Parish Big-3.
This current Celtics team has the THREE and may have the EIGHT, but definitely does not have the ONE in the 1-3-8 rule-of-thumb.
I do agree that the current "Big 3" doesn't have a sure-fire HOFer. However, I think it is more important for the One to be an MVP caliber player during the championship run, than just having a washed up HOFer. Thomas finished in the top 5 in MVP voting last season, and with the addition of Hayward, he may improve even further. Maybe his scoring numbers go down slightly, but his assists may increase.
Are they a Big 3? I would say yes. I todays NBA, they have 3 max level players close to the prime of their careers (with Horford towards the end of his prime). If the team is healthy, there is no reason they shouldn't finish in the top 2 in the Eastern conference. The expectation is to at least make the Eastern conference finals, if not the Finals next season. Not many teams in the NBA can make those claims.