A swarm robotics project to search to find a particular location on the ground.

Hi,
I am trying to implement a 3-bot Swarm Robotics project that will try to find a particular location(a white spot) on a black background. I am trying to implement the task shown in the video below:
I tried contacting the person who posted the video for guidance but did not get a reply.

1. The 3 bots must move in a coordinated search pattern to look for the spot.
2. Collisions with the surrounding walls and each other must be avoided.
3. Whichever bot finds the spot first must indicate to others that it has been found.
4. The other bots must reach the location as shown in the video.

My Problem : I want each of my bots to know the distance between itself and other two bots. What are the sensors that I can use to achieve this? I thought about using the ultrasonic distance sensor(HCSR-04) but how can I make it differentiate between a wall and another bot?

Any help is highly appreciated. Thanks!