I wasn't aware there were major similarities between any of them.
Electro is a capacitor, absorbing and discharging electricity, and manipulating electrical equipment, a specific power for electricity. Magneto has power over the electromagnetic spectrum, focusing primarily in magnetism. Doctor Doom is a mystic with supernatural abilities that wears some badass powered armor.
Who was first is an easy one, Doom beats Magneto out by a year, you can find out such incidentals on the Marvel Universe wiki, as well as detailed explanations of the characters.
