Thats a very good example, just like building dargahs or praying on graves is forbidden in Islam but still many muslims do that, the same way worshipping of idols is not taught by hindu scriptures, but still hindus do that.
Every religion goes through currpution phase, Chirstianity was currupted during medivial times, hindusism got currupted much before that and these days Islam is getting currupted by the misuse of the word is Jihad.
Why I say this, I love to do comparative religious study, I have studied books on Skihism and hinduism, buddhism, but not on Chistianity.
Om does not mean trinity. Its a sacred sound in Sikhism, hinduism, jainism, and buddhism. Its the sound from which the universe was created. Ek Omkar means One God, and that is believed by all above mentioned religions..