What Quadcopter should I buy for formation flight?

[Shawn954]

As the title stated for my school project(formation flight), how do I begin with?
I need at least 3 UAVs to stay in formation.

1st - What UAV should I buy? I have a budget of <$400 for each one. (Must be programmable for them to stay in formation)

2nd - How do I program it for them to stay in formation?(eg. staying in a triangular shape position)
what do I need? any extra parts?

This is for my school project so any help would be appreciated
Thank you!

 

[RENOV8R]

There are programmable flight controllers (i.e. APM Mission Planner) that you can pre program flight paths. The problem is that they are controlled by GPS and for obvious reasons, the government doesn't allow accuracy of less than 1 or 2 metres, so you wouldn't be able to have a very tight formation.