Hi Mark,

I believe that the bottle (containing high pressure air) is used to start the #3 engine when a ground power/air cart was not available or not working. Then, once #3 was running, you would use the blead air from #3 to start the remaining engines. The light would come on and remain on because you discharged the bottle and ground crew would have to recharge or replace the bottle again.
Normally, you would use low pressure (ground cart) to start #3. The bottle would not come into play then.

Maybe Lou are some one else can correct me if I"m wrong, as I'm not a real pilot.

Rick