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Exhaust profile , continuous flame, lot of factors work simulations so you just can't put a valve, if you remember the throttling of even solid boosters is done by reducing the exhaust area or the exhaust profile

Merlin engine of Falcon heavy have how throttle range too but what they do is ,they only give fuel to one of the engine of the nine and you get the result intended

Or you can achieve reusablility by having the whole cluster of booster and first stage being landed on lahch pad using single of three engine,