As you can see, the sign was vandalized Friday with an image of a dairy cow taking over the middle of the first 'O.' Law enforcement sources tell TMZ ... 3 people have been arrested for trespassing, with a police helicopter helping officers track the suspects as they made their way back down the hill.

We're told the alteration to the sign was done as a joke, and not as a protest for or against bovines.

There's a band out of Los Angeles, called Junior Varsity, appearing to take credit for the vandalism. JV just released its debut single, "Cold Blood," which features a dairy cow as the cover art and their social media accounts have been telling folks to "Look for the signs."

It's the second time in as many months the Hollywood sign is getting an illegal makeover ... back in February, model Julia Rose was arrested for altering the sign to read, "Hollyboob."

Ya might say folks are milking the poor sign for all its worth.