Hi,
I made a robot some years ago using different sensor as your doing.
My experience with the SR04 sensors is that if they are too low (close to floor) then they
will give false reading.
If you have 3 SR04 in front, maybe the echo of 1 interfere with the other. Personnaly I use 1 in front and 1 in back.
If the robot move in a corridor (2 walls parallèles, 3foot distance), the echo will bounce many times
on the 2 walls and give max distance.
I think for small objects, sonar is to be monted on a Micro servo that turn often.
In general IR sensor are good for small distance (<15cm) and help to avoid contact with object (if the robot is not too fast).
I think I will start again playing with my robot!!
Have fun with your project; you certainly will invest many hours in it.!!!
Nota:please excuse my english