So, no takers on this 'What is it'?
This bush is commonly called Pacific or Western poison oak, or just poison oak. It can grow as a shrub as in my pic or as a vine trailing up even very large trees. It's found though out California's valleys and lower foothills. Thankfully it doesn't do well in colder winter climes so none of this around our place.
In the Spring it is easily recognized by its bright green 3 leaf clusters (hence the rhyme) that turn deep red in late Summer or early Fall. This year there are a lot of these plants that have already turned red and in this example even brown as they drop their leaves becoming just a cluster of sticks. The leaf drop doesn't usually happen until early Winter but things are different this year.
Why should you leave this plant be? Many people are affected by the plants' oils that can cause red itching patches that if left untreated can develop into itchy blisters. The plant oil can be spread beyond the original contact to the face or even eyes, for example by tying shoe laces that have been hiked through some plants and then adjusting your sun glasses. Hospitalisation can be required in some cases.
In our younger years when we hiked in poison oak habitat we would always wash all our clothes in hot water and shower with soap specifically designed to wash away the oils. Worked most of the time...