Longer middle stem,stay away from them.
This refers to the middle leaflet having a notably longer stem than the two side leaflets and is a key to differentiating it from the similar-looking Rhus aromatica-fragrant sumac.
Red leaflets in the spring,it's a dangerous thing.
Sometimes in the spring new leaflets have a red appearance.(Note that later,in the summer,the leaflets are green,making them more difficult to distinguish from other plants,while in autumn they can be reddish-orange.
Side leaflets like mittens,will itch like the dickens.
This refers to the appearance of some,but not all,poison ivy leaves,where each of the two side leaflets has a small notch that makes the leaflet look like a mitten with a "thumb."