A few years ago I had a Pacer, and was fortunate enough to have a large group of flying buddies. Every Saturday some of us would fly together. There were three other Pacers and sometimes as many as five Super Cubs.
We fly out of DVT, and it is an extremely busy airport. If we launched individually, sometimes it would take half an hour to get everyone airborne. This meant a lot of circling, and sometimes we just couldn't find each other. We decided that a formation launch was the way to keep the group together.
For a long time it worked great, we taxied as a group and took the runway together, then, one at a time, took off. The drill was wait until the guy in front was airborne then go. With my trusted friends we got to the point where we could take off in tight fingertip formation two at a time. (One plane slightly ahead, but basically side by side.)
There were two incidents that got me straightened out on the whole formation thing. The first happened when there were three of us. I was the #3 plane so the plan was for the other two to go first then me.
It was busy, and I could tell the controller was geting impatient, so instead of waiting until they were airborne I firewalled it. I thought I'd just keep the plane on the runway, gain a little extra speed, and zoom climb through the wake.
The ground roll was normal, but as soon as I rotated I hit wake. It totally overpowered the Pacer. I can clearly remember watching my hands roll the yoke to the right stop, and pump the elevator control back and forth, to no avail. The yoke was dead, the rudder pedal was pushed to the floor, the engine was at full power, and I was a passenger. The plane continued to roll; I just sat there, thirty feet off the ground wondering what would happen. Visions of cartwheeling raced through my mind, and I became aware of a funny growling noise, which turned out to be coming from me.
Then it was over; I don't know if I came out the top or the side, but I was free.
Looking back, I knew I would hit wake, but I was completely unprepared for how powerful it would be. Flying sailplanes, and in other formation flights, I'd flown through wake many times, but always at cruise speed. It sure makes a difference if both planes are slow.
I don't know what the take away message here is. Clearly I don't recommend taking off before the plane in front is airborne. At the same time, I don't want to discourage departing as a flight. It's a great way to keep a group together.
I'll leave it to you to draw conclusions.
I'm getting long here, so I'll share my other formation story some other time.

John Crocker crashed his P-51 at Boeing Field
in the 1980s due to hitting the wake of a Sea Fury
he was taking off in formation with. The fact that
the Sea Fury's prop turns the wrong way may
have complicated things, but in the end, it still
knocked an 8000+ pound, 1500 horsepower
Mustang to the ground.

I've also done the formation take-off thing (up
to four of us have taken off in formation, thankfully,
on a nice big-long, wide runway). I guess you just
have to be prepared for the wake(s), and ride 'em out!

Bela P. Havasreti

The formation takeoff's I've watched took off as 2-ship elements in staggered pairs. Close enough & offset enough where I would think wake turbulence wouldn't be a factor for the second of the 2 ships. But maybe for the following element. Maybe with the 3-or more ship takeoff you describe have the first pair start their takeoff roll with partial throttle, as to actually break ground (& start creating wake turbulence) farther down the runway. Then the following ship(s) would be off the ground & flying before the wake, and would stay above it?
I often take off pretty much right behind my flying partner, and either stay to the side of the runway (if it's wide enough), or else make it a point to break ground either before or after where he did, as not to cross his wake. Also dogleg a bit right after takeoff, to further avoid wake.

Eric

Eric you're right, if you stay close to the plane ahead and slightly to the side you'll never have wake problems. I got into trouble because I took off behind, and more importantly between, the planes ahead. Worse, I knew I'd hit wake and underestimated how bad it would be. To fly safely in close formation takes training and group disipline; We had neither. I would only recommend loose formations and in trail take-offs like I described above. It's safe that way, if you want to get closer take a formation class.

Been there Speedbump. Took off behind a Champ once in my Supercub and at 200 feet all of a sudden was just along for the ride. It pushed the nose of the Cub down and to the right despite full power (41 Borer), full left aileron and full left rudder. Skidding along sideways to the ground unable to do anything except look at the infield gets your attention. The moment it got out of the wake I had full control but now I always skitter off to the side if I'm departing with another airplane.

Brad

(who'd a thought a 90 hp Champ was capable of creating a wake like that!

Several years ago a friend of mine encountered some severe wake turbulence landing at Seattle's Boeing Field. His attempt to go around ended up with his Cessna 150 hanging by one wheel from some powerlines adjacent to the runway. Some of you might remember, this one had national TV coverage. "Highwire Mike" Warren was killed a few years later in the crash of a Yak 52 near Bellingham, we still miss him.

Eric

I once flew too close to a goose (didn't see him until too late) and he passed just off the wing tip down the right side. Once past the wing he exploded in an erruption of feathers. Wing tip vortices and wake turbulence are pretty powerful things. I have a feeling my lil' ol' Maule would suffer the same fate as the goose if I passed too close to larger aircraft.