Training Room (Luigi's Mansion 3)
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There are three treadmills and exercise bikes where Luigi can examine by blowing or sucking to peddle, and run on the treadmills. One of the exercise bikes has the key while the other two is filled with bills and coins. To obtain the purple gem, the radio must be set to Channel 3 by flashing it with the Strobulb three times. After doing so, two Golden Goobs and a Gem Goob will be on the treadmills.