Hi Buzz Bee,
Don't know if this will be of any help, but like you my husband got the same benefits.
DWP told me that all three benefits had a different criteria and one should not effect the other, ie if you did not get ESA/IB, you could still get DLA and Industrial Injuries.
Therefore, try to concentrate on your ESA as you have limited time to get the form back and with the brilliant guide lines of how to fill out your ESA form, you may be worrying needlessly and i am sure all on the forum will try to help should you get stuck!!!
Hope this helped a little:).
Radhika.