swimming ?

Depends on the intensity. In a race, probably every stroke. Lower intensity stuff in training, I usually breathe bilaterally, so every other stroke.

I breath on the 3 

I normally breathe every 2nd on my right. 

I can breathe on my left, but not as good at it.  Something I've been working on specifically in practice.

Mark

All depends on the intensity...

On the 3, 4, 5, 6 and 7.  Though like others have said, it's intensity dependent.  I try to do all of my easy workouts on a 4/5 breathing pattern - it's taken me years to build up to this and if you are a beginner, focus on every 2 or 3 for a long time.  Occasionally I will do the first 50 yards on 7/7 then roll back to a 4/5.  Fast workouts, which I infrequently do I usually fall back to a 3/3.

The surprising thing I find when you train in many different breathing patterns and get to know them well, is that when your body is 'run down' or 'getting sick', you can't maintain your usual comfortable breathing pattern.  For example, even if I'm not conscious of it yet, if I can't maintain at 5/4 pattern on an easy workout, I know I need to keep the workout light and to get some sleep for the next few days.  Or make sure I have had a rest week.

I find breathing patterns fascinating as a barometer into your well-being.

Every stroke. Unless I am specifically doing drills which focus on bilateral breathing. 

Every right arm.  Seems to work better for me to keep air going in and out more regularily

To each their own though.

I'm like the others who say it depends.  I don't give my breathing pattern any thought at all.  When swimming easy, it may be every 5 strokes, alternating sides.  As the effort level increases in open water where I want to see what's on both sides of me, I will find myself in a 2, 2, 3, 2, 2, 3...pattern, breathing twice in a row on one side, skipping a stroke, then twice on the opposite side.  In races, it frequently becomes every 2 strokes, breathing on the same side to avoid sun, waves, or splashing from another swimmer on the other side, and getting enough air.  Sprinting, there's no breathing.