Powering up legendaries

Usually 3 at lvl 30 would be better. The exception can be in solo or short man raids. For example, if you want to solo claydol you better have 1 kyogre at 40 than 3 at 30.

My feeling is that the best lvl is the first breakpoint after lvl 30.

I always try to power up one of each legendary to level 40. Then the rest to level 30 or so of the good ones.

In my opinion, that really depends on your stardust situation. For me, rare candy is the limiting factor for legendaries but dust is not. I can easily have 5 lvl 40 Gyarados (which would be a waste of stardust, I know but I have saved so much by now) which are only slightly outperformed by a lvl 30 Kyogre. Therefore, I prefer to have one Kyogre at lvl 40 instead of 3 at 30 (right now I have 2 at the last Groudon/Regirock breakpoint, lvl 36.5, and a third at lvl 30).

It entirely depends on your situation. For instance, someone whose been playing since day 1 probably has a pretty solid team of vaporeon and Gyarados. As well as a bunch of high level dragonites. That person would be much better served powering 1 Kyogre and 1 Rayquaza up to lv40.

Meanwhile, a newer player who might not have a team of 6 powerful water/dragon mons might be better off powering a couple Kyogre/Rayquaza up to lv30 (especially if they are already weather boosted).

My opinion is that you should build your type base wide (at least 6, but up to 12 deep), then once you are there start building up.