Trying to slot in WTA players alongside ATP ones to make a "big 3" is jumping the gun.
Serena is currently a "big 1" and the only one that can be compared with Federer/Nadal/Djokovic. For the rest it's too soon to tell - we need a few more years to see if this is a big 2/3/4/whatever.
No WTA player made at least the QF of all 4 slams either last year or in 2011, so we've not yet seen the situation that the ATP have had for years.
The key problem is not the current crop - it's the generation of the same age as Nadal & Djokovic (Ana, Jelena, Sveta, etc) that mostly faded away, and have left the field to a mix of younger players & some late bloomers (like Sam & Na)
. The younger ones have years ahead of them to improve and progress - or to also fade.
In terms of achievements, Azarenka is currently more like Murray - gradual progress over a few years and now getting the temperament to win slams. But still too early to judge how the coming years will play out for her.
I'd see Sharapova as more comparable to a player like Hewitt, who's had a very good career (some slam wins & the number 1 ranking) but after early success couldn't live with the multi-slam greats when they hit their stride. It seems she can't adapt her game enough.
If ATP players like Berdych, Del potro, Tsonga and others hadn't had to contend with three great players like Federer, Nadal & Djokovic, and instead there'd been just one great (as with Serena in the WTA) their careers would look better
EXACTLY. I'll never understand why people blame everything on the young generation. We are talking about 21, 22, 23 year olds at most, who have years ahead to win more Grand Slams/fade away/emerge/whatever, but somehow nobody remembers that the likes of Jankovic, Zvonareva, Ivanovic, Kuznetsova, etc., are the ones who should be filling that spot right now, dominating the tour, beating Serena, winning the Grand Slams.
I mean are Goffin, Dimitrov or Nishikori bashed because they're not part of the "Big 4", or because they've never come close winning a GS? No, because they're still young, and they have a bunch of 25, 26, 27 year olds in their primes winning all the important titles. The problem in the WTA is that we don't have those 25, 26, 27 year olds. We blame the youngsters but it's not their fault that Ivanovic and Kuznetsova have become irrelevant. They are actually better ranked, have better results and even win GS, but still get all the criticism.
So if we're gonna bash a generation, let's bash the one that came right after the "Golden" one but failed to fill its gap. But even so I realize it'd be an unfair thing to do, because filling that gap is not easy for any generation.
To answer the question, there's a Big 1, followed by a Big 2, followed by the rest of the field led by Li and Radwanska.