How does that old Boy Scout saying go? Leaflets three, let it be. Barry White, disco in sight?

Answer 1

Leaflets three, let it be; Berries white, poisonous sight.

**Here's one for snakes**
Red on yellow, you're a dead fellow;
Red on black, you're okay Jack.

Answer 2

Close Leopold, but not quite.

Leaflets three do refer to a single leaf of poison ivy/oak. Toxicodendron radicans. The oil which causes itchyness is called urushiol. (The urushi tree of japan has the same properties, yet they use it to make laquered bowls.) The itchiness itself is dermititus.
The leaflets themselves can be quite diverse in morphology. Always, though, the middle leaflet is offset slightly by a small-ish bit of stem. (Skunk sumac is easily confused with poison ivy, but it lacks the slightly detached middle leaflet.) The main differences between poison ivy and poison oak are their methods of attaching themselves to support; both can occur as either vines or shrubs.

And in general, plants with clusters of drooping white berries are toxic to eat- regardless of whether or not birds eat them.

Answer 3

The leaves of three refer to poison ivy. Ivy always has three leaves. Two leaves look like mittens and the middle looks like a mitten with two thumbs. Ivy contains Dermatitis, that is what makes you itch.

The 'berries white' part refers to the Snowberry bush. This plant has white berries that are suprisingly enough, are quite poisonous.