Three scoring lines is definitely a pipe dream, the offense would be watered down.
The reality is, that even though we have 7 talented forwards, only Hall and McDavid are capable of anchoring a scoring line at this point. Draisaitl may get there too, but I won't include him yet due to his small sample size.
Pouliot, Eberle and Yakupov are complimentary players. They can produce offense, but also have many nights where they accomplish very little, even playing with our best players.
RNH is somewhere in between at this point. Better than a complimentary player, but his impact on a game-to-game basis can fluctuate a lot. He always looks very smooth out there, but some nights he looks elite, and many others he just looks ordinary and ineffectual.
Probably 4-5 of the above 7 guys will be here long-term (Hall, McDavid, Draisaitl, and 1 or 2 of the others). Lines 1 and 2 should utilize those guys. There just isn't enough ice-time to go around with inevitable penalties etc to have 3 scoring lines. And as I've stated above, only a few of these players are elite, the rest are just good.