TTT turbo sessions

There is little point in starting training  for anything which is in 3 weeks..

For a TTT you would be best doing over/under sessions rather than hard/easy intervals.  You want to be aiming to hit the front of the line and hold your speed. This will result in a rise in power as you hit the wind, but you don't want jump into a sprint and gap the rider behind you. It's all about being smooth. When you peel off, you don't drop down to L2 pace or you'll be ejected off the back of the line, you have to just ease off the gas and drift back down the line. If you let your relative speed to the bunch drop too much you have to sprint to latch on. Do this too often and you'll explode.

As such, the over/under workout is where it's at. You have to be riding above threshold when you're on the front, and you need to be able to recover while riding close to threshold.

Ride in a line as close as you dare. First rider peels off drops to back. Eventually you do your turn at the front. Don't go too hard or too long on the front or you won't last the course.

Training, do interval sprints of a minute at full power. 1min hard, 1 min recovery Up to 10times, if you can last that long. Do 2 sessions a week and one longer ride to keep endurance. With only 3 weeks this is the only way to build a fitness for such an event. You have said you have a decent base fitness. You never know you might be fitter than your team mates and then it will be easy

I'd spam Sufferfests for a few weeks. You can get a week free at the moment if you have a Mac and download the app, which gives you access to all vids. 5x5 squat sessions on top of that, maybe twice a week. 

Would be a tough three weeks of training but it would go in fast and you'd be putting out your best effort. Probably ditch the squats if didn't have weights at home. 

My question is why ride a turbo when summer has finally arrived?

I'd have thought that the most crucial thing is to practice riding with your teammates.