The only stipulation is on "STE" front row players (youth laws are slightly different) and law 3.13 is important for me.
Scrums will become uncontested if either team cannot field a suitably trained front row or if the referee so orders.
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Logically extrapolating from 3.13 we have:
Scrums
will be contested if both team can field a suitably trained front row.
If there are 3 players who are STE to be able to fulfil LHP, H and THP on the fied or ont the bench then they play and in those positions. If they c my LHP goes out on the wing to create a non-scrummaging FR the referee should say "either you scrummage or you are off the field as "permanently replaced (injured).
the following suggest this is the correct approach:
3.10 Prior to the match, each team must advise the appropriate match official of their front-row players and possible front-row replacements and which position(s) in the front row they can play. Only these players may play in the front row when the scrum is contested and only in their designated position(s)