Journals have gotten much better about shortening review times, with many journals averaging something like 40-60 days... which is actually more like two to three months (do weekends count?). But that is an eternity for junior faculty, and the reality is that rejection means that one goes through this spin cycle more than once to get most articles published. I enumerated the many rejections my published work had received before getting published (my work got better via the reviewers' recommendations and/or I found journals with more sympathetic* reviewers).
* Sympathetic might mean those with lower standards or with fairer expectations or less picky due to reviewing for less highly ranked journals....So, to mitigate the risks entailed with the odysseys that most articles go on, it makes sense to have a bunch of stuff out there in process while working on the latest pieces. Indeed, of the three pieces I currently have out under review, two are contributions to special issue submissions which move, like any edited volume project, as fast as the slowest contributor (just like naval convoys in WWI/WWII moved as fast as the slowest ship).
Tis a good thing I have flipped another project (very long in gestation) over to a co-author for final revisions before it goes out. Next, I turn to yet another piece that has multiple co-authors and has taken much time to get to a submittable stage. This time, I am the obstacle, making this project slower than it could have been. I am sure one of my co-authors will be reading this and think: he should be revising our piece. And the answer is: yes, I am.
Anyhow, with low acceptance rates, sometimes long review processes and often fickle reviewers, it makes sense to place as many bets as one can. Three at a time is not a bad target, but, of course, requires much time, work and creativity. I used to wonder if I could have a second or third research idea. As careers progress, having enough ideas is not the problem but having enough time and resources to do the work is.
Sometimes heaps of work on one project means falling far short of the three under review goal. It is just a goal and not a requirement. But it is not a bad habit to try to get into.