Not sure how reliable UA's flight is. If it was NZ (flights are rarely delayed departing SFO), then 3 hours for separate tickets would be okay. This gives a buffer for small delays on arrival, clearing immigration & customs (can be really busy at times), transfer to the domestic terminal (10 minute walk in the open or allow 20 minutes if bus needed due to bad weather) and check in at domestic terminal for NZ or JQ (jetstar) before the flight closes.