There is an app (or apps) that will allow you to set up more complex repeats than the built in Calendar app. Sorry, don't remember the name but I'm sure some time searching the app store would turn up something.
There are only limited manual options for repeat on iOS calendar if you enter your appointment manually. A workaround is to use Siri. Siri can set appointment repeats any way you desire (every other day, every three days, every three weeks, every 5 months, etc)