Hey Theo,
Like others have said, mid-March can be tough to predict. Could be snowing. Could be 70 and sunny.
If you and your friends have your hearts set on climbing one of the Flatirons I'd suggest trying the Second Flatiron first, and if you guys fly up the Second, give the First a shot.
Your friends being non-climbers means you'll be pitching out the entire climb, instead of soloing or simul-climbing large sections, and it likely means they will be slow. Pitching out the whole climb can take a lot longer than you'd think. If you've never done the route before, you can lose a lot of time to route finding and trying to find a suitable spot to build an anchor. First time up there, it really does just look like a sea of sandstone, and there's a fair amount of bad rock if you get off route. Not to mention the runouts are legit. There are a lot of spots where you could be looking at 80-100ft falls, so simul-climbing with non-climbing partners would be horrifying.
Seriously, not trying to be a wet blanket. You know your groups ability a lot better than I do. It's just the First Flatiron gets underestimated all the time, and in March, it gets cold fast once the sun goes down. If you have your heart set on the First, definitely bring extra layers (even if it is 70 and sunny), extra water, and head lamps.
Whatever you choose, I hope you and your friends have fun!