If you have 8 feet or Fina regulation 1.5 meter lanes then 7 to 9 will fit well.
I don't have a good reason why wider lanes makes it feel more comfortable when swimming.
With three lanes, it depends on the crowd and the spread of abilities. We have triathlon workouts that include people with cruise 100 speeds from 2:05 to 1:15, it is difficult to work them into 4 lanes without having them all over each other during sets. I think with three you will need a relatively tight distribution of athletes.
Splitting the groups up into faster and slower (worded delicately) for two sessions can help with that.