It's best to have a reliable static in order to make reasonable progress. Every time you have to pug someone, it's someone new that you have to train to your particular strat. There's a lot of minor variations that, while mechanically easy to adjust to, can throw people off enough that they aren't performing at 100%. My static was having serious roster issues going into T6-8 (people that didn't show, etc) and our progress was completely stalled at some points until the problem people were replaced. Basically, a reliable 3 isn't really enough. :|