Hello & welcome
(1) I think you can do both at the same time qt & tank cycle or you can wait till cycle is over
(2) you can qt more than 1 fish, not sure about 3 tangs all at once
(3) it would be a good habit to qt every fish prior to adding to main tank
(5) you could some hardy fish chromis or a pair clowns, then add your other as the tank matures
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