Are the phrases below correct?
1)a bed to sleep in
2)a bed in which to sleep 
3)a bed where to sleep
4)warm air currents to soar on
5)warm air currents on which to soar
6)warm air currents where to soar

The following phrases are not correct:
3)a bed where to sleep
6)warm air currents where to soar

We don't use "where" this way in English. To use "where" you would have to add a personal pronoun. E.g., "A bed where I sleep." The other sentences are correct.

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