The setting of three is arbitrary and has been chosen as the most effective OOB configuration. You can tune this (along with other cache related parameters) using prconfig file and "cache/probationthreshold".
As I understand, even if you set this to 0, it will still result in rules having to be accessed twice before they are cached. The probation policy keeps rare rules from filling up your rule cache. First access puts a rule on "probation" to be cached, second access will see if subsequent access count has exceeded "cache/probationthreshold".