1 d4 2 c4 . . . when 3 Nf3?

Somewhat of a newbie.

In a Dan Heisman Novice Nook -- the Most Common Opening Inaccuracies (https://web.archive.org/web/20140714190611/http://www.chesscafe.com/text/heisman62.pdf) -- he writes (giving an example of the Slav: 1 d4 d5 2 c4 c6)

There is nothing terribly wrong with 3.Nc3, but most grandmasters play 3.Nf3, which avoids the Slav Gambit 3.Nc3 e5!? and aids in subtle transpositions. The general rule is that in games where Black has played either …c6 (Slav) or …dxc4 (Queen’s Gambit Accepted) and may play
…b5 and possibly …b4, it is better to delay having the knight on c3. So the main move is Nf3 first.

OK, so I get the first reason: 3 Nf3 protects against 3...e5

I don't quite get why white's 3 Nf3 is superior in relation to black's threat of ...b5 and ...b4

Any thoughts?

Off the top of my head: b5-b4 forces the knight on c3 to move... sometimes white doesn't care and sometimes white does.

Nc3 allow black to play dxc4 in Slav and a6 in QGA followed by b5 b4 which put white into a playable but very uncomfortable position

The b4-b5 tries by Black to hold onto the pawn aren't all that good. I think the lines he refers to here are probably ok for White, but can be dangerous for a "Novice" (the intended audience). You might find this to be a reason to actually play it.

Funny thing, I'd just gone through a couple of these old Heisman posts and was reading about another "rule" on not playing Nc6 in double d pawn openings (iirc, that's the wording). Well, that would lead to Chigorin variation or Chigorin Defence ( I never liked as Black).

Here's that one (pg 6 more good opening guidelines #3):

https://web.archive.org/web/20140626203522/http://www.chesscafe.com/text/heisman31.pdf

But to add to reasoning before on early Nc3...what does it do for you? Not much compared to Nf3 (which also protects d4 and the d-file from opening up on you).