Three INCHES. This is not necessary unless the undergrowth is very dense or your humidity is high enough to cause mold worries.
You can start with MH to reduce stretch then switch to HPS after 2 weeks, but if all you have is HPS, and you have appropriate watts for your space, stretch is going to be mostly determined by genetics, although controlling night temps so the day/night difference is 10'F or less will help too.