Hi! Sounds like fun! I home schooled and am a retired school teacher, so maybe I can help.
PE is about getting physical! An obstacle course is a great idea - you can do it on any playground. Yoga is wonderful, 45 mins may be too long for the 3 year olds, but if you keep it moving along, they should be fine.
I would pass on kick boxing - just not safe. Unless you are a trained professional with the proper equipment, then go for it.
Other ideas I have used in the past:
* swimming is always a hit - and the lifeguards are there to support you.
* "training" for a triathlon - the kids loved this - running, biking and swimming. * We also made up other "triathlon" activities - we did skipping, jumping, hopping anything that was age appropriate. (3 year olds might not be riding bikes yet!!!) Each time we got together we would time the "events" and see if we improved on last time.
* any kind of "races" - rolling down hills, skipping, hopping, lunging. I would always line the kids up in "height" so that it was fair.
* dance - any kind of dance, any kind of music. Line dances are a hoot and any age can catch on
Most importantly stay safe and have fun!