I would suggest getting 2 each 3x3 tents. More tents means more equipment, which means more money, more rigging, more maintenance. The price jump from a 3x3 light to a 4x4 light is pretty steep, usually an increase of 0-0. Either way, two tents gives you one flowering tent and one seedling/veg/clone tent. Only way I would recommend a third tent is if you want to use it as a drying room, in which case I would recommend getting a 2x2 for drying.
In a 3x3 tent you can easily fit 9 plants with each having 1 sqft. of space. Keep in mind to observe various legal restrictions. Many states restrict how many plants can be flowered at the same time. Most legislation will also classify clones and seedlings as plants, although some states only put a cap on mature plants, for which the definition will vary.
Let's say you decide to give some extra space for each plant in your flowering tent and flower 6 plants in a 3x3. If you're pheno hunting, then you can pick a clone from 6 mother plants and flower the clones to decide which to keep. You could toss a few of the mothers you didn't like and then fill their space in with new strains. Or keep a small stable of 3 0different strains as mother plants and rotate which strain you want to clone and flower 9 clones each run. You could pop 6 seeds in one tent and once it starts flowering, pop 6 seeds in the other tent, staggering your grows. A lot of this depends on the law and what limits are in place, but I would say that you're best off with 2 each of 3x3 or a single 3x3 for flowering and a larger 4x4 to move around in more for veg. Veg doesn't need as much light as flowering does, so you can get away with a 3x3 light in a 4x4 veg tent.
Either way, a 3x3 or a 4x4 will provide you with a lot of bud, if fully utilized. I yield about 4 oz (dry) in a 2x2 cabinet, which is typical and expected. General rule of thumb is 1 oz per sqft, but this of course skews a little bit depending on the plant's genetics. So you should expect ~9 oz from a 3x3 tent and ~16 oz/1 pound from a 4x4.
Another thing to consider is automation. BUY SOME SMART PLUGS! Any brand is fine, but I use the cheap Kasa smart plugs and they are fantastic. No need for a timer if you have smart plugs and changing your light schedule is easily done in your phone. Something specific about Kasa smart plugs is that I had issues with the timer not operating properly when using Google Home to control them. The schedule works flawlessly when I use the Kasa app, but Google Home would sometimes not properly trigger the plugs to turn on/off.
IMO, it is better to have dumb equipment plugged into smart controllers than to have equipment with smart controllers built in. If the controller fails on your smart equipment, you're screwed. If the controller fails that your dumb equipment is plugged into, then you can easily replace the controller and run it dumb until the controller can be replaced.