Okay, mathy people, can anyone help me figure out how to make this work?

I'd like to have two methods of stat generation that give, on average, equal results in the end.

The first is more or less what rolling for stats looks like right now: 4d6, drop one die of your choice, then add (for a typical character) +2 to one stat and +1 to another.

The second is the challenge. I want to create an option for rolling 3d6, but with several sets of +2/+1 ability score increases: one free floating, one for race, and one for class... but I don't know if that's enough to equal that extra die-drop-one. I feel like it's not.

Can anyone help me figure out how to balance these two options?

Since some people got this wrong... I'll set it straight for you.

Yes, 3d6 averages 10.5, but 4d6 drop lowest averages 12.24 (NOT 13! where did that come from???). [As

@Hriston said.]

So, the difference between the two is less than 2 points, about 1.75.

Yes, that is for each of the six abilities, so adding those differences together is 10.5 (hey, fancy that... the average of 3d6!).

**Since each +2/+1 ASI combo is 3 points, you could do three +2's, three +1's (must all be different abilities--so each ability gains a +2 or +1). That gets you 9 points, so you only need about 1.5 more, which could be the +1 for race and +1 for class.**
For example, suppose your 3d6 rolls were (average 10.5): 8, 12, 12, 9, 14, 8

Adding three +2's and three +1's:

10 = 8 + 2

13 = 12 + 1 (EDIT: removed if only two +1's are allowed)

14 = 12 + 2

10 = 9 + 1

15 = 14 +1

10 = 8 + 2 (EDIT: removed if only two +2's are allowed)

Then you have two +1 floater for race and class:

10

14 = 13 +1

14

10

16 = 15 +1

10

Final array: 10, 14, 14, 10, 16, 10, average 12.333 (slightly better than 12.24 for 4d6 drop lowest).

So, there you go. Hope it helps.