I see what you're saying, but the player's handbook says, on page 15:

" Each time you gain a level, you gain I additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up)"

To me, this looks like it's saying that you add the constitution modifier to each individual hit die. So if you have 3 hit die, you would get (3*Con + 3* Roll)

In other words, for a Sorcerer with a +2 Constitution Modifier and rolling 5 and 3 for the second and third levels respectively:

1st Level: 1d6 = Max + 2 = 8 Total

2nd Level: Roll 1d6 = 5 + 2 = 7 + 8 = 15

3rd Level: Roll 1d6 = 3 + 2 = 5 + 15 = 20

Am I just wrong about this?