Question on Sets/Reps

when at the gym and going from machine to machine, i know your only supposed to wait 30-90 secs between sets, but does this mean you wait 30-90 seconds before moving to the next machine? or does it mean you do all 3 reps on the first machine waiting 30-90 secs between sets,then move on to the next machine?

i do full body when i go to the gym, so i use basically all machines, but was just wondering which way is more productive on gaining lean muscle mass

the way i been doing it is doing my 15 reps on first machine, wait 30 secs, move on to the next machine and ext. then starting back over at machine 1 once I've been through all the machines doing that 3 times 

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