My understanding is that if FR players are available, they must be used - regardless of whether it's contested or uncontested.
I agree, but is there some strong regulation in law book or its just depends on, as you said, understanding?
Covered by 3.6(e)
3.6(e) If they are available, a team must have three front row players in the front row at all times. In an uncontested scrum, only when there is no available front row replacement or substitute is any other player permitted to play in the front row.