Nothing will happen to any of these Gods. The brahmastra was ineffective even with Anjaneya (reference. Sundara Kanda) and Garutmanta(reference. Amritapaharana) These two heroes chose to respect it by pretending to be bound by it and offering a feather for the Astra to nominally serve its purpose. So, the brahmastra will be ineffectual with the trinity too if someone does indeed exhibit the temerity to launch it against them.
Once Lord Shiva wondered if I am the destroyer can I destroy Brahma and Vishnu too?
Lord Vishnu came to know about this so he and Brahma visited Kailas.
Lord Vishnu said
Thou art Mahadeva the eternal origin of all seeds, please share whats on your mind with us as we are the same entity from roots.
Lord Shiva shared his thought and they all three started discussion.
Lord Vishnu was very curious what would happen so he insisted Lord Shiva to apply for the next on Lord Brahma.
Lord Brahma said
Please destroy me o father
Lord Shiva started destruction and only ashes left in place of Lord Brahma. Suddenly a voice appeared
There is creation of ashes so where there is creation I exists.
And Lord Brahma menifested from Ashes.
Now there was a turn of Lord Vishnu. Lord Vishnu said
O almighty Lord Please destroy me but keep destruction continue on my ashes until the ashes also destroyed.
Lord Shiva did the same and ashes appeared in place of Lord Vishnu because of Shiva's power. Shiva continued to destroy even ashes. The ashes got very small even smaller and then at some stage ashes left effects.
After some time noise appeared
Where there is preservation of even a particle I exists.
And Lord Vishnu appeared from ashes. Lord Shiva now knew that so he decided to burn self in ashes. Lord Vishnu and Lord Brahma furiously watching to him because Shiva decided to kill himself.
They both tried to stop him but he did not listen and burnt him. Because of this action All of three became ashes. Everywhere was darkness only. The whole universe became dark.
Suddenly voice appeared
There is creation of ashes so I exists
Lord Brahma appeared.
There is preservation of ashes. So I exists.
Lord Vishnu appeared.
Preservation and creation is not possible without Destruction So I always exists.
Lord Shiva appeared and put ashes all around the body.
I think Brahmastra is only a concept. Even if such weapon exists it will not affect almighty deities. And Even if affects nothing happens.
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