What's pictured in Oxer's photo is what I call a placement rail - you put it one stride out from the fence to ensure the horse meets the correct take off distance. Setting a placement rail is a little trickier and usually requires someone on the ground to help.
Oxer is right, though - for a 3' fence, the grouind line should be rolled out about 18". Since you're jumping stuff lower than 3', stick with 12 - 18" out from the fence.
A series of trot poles in front of the fence teach the horse to hold a steady pace to the pace of the fence and also dictate the take off spot. For a small horse or one with a fairly short stride, 4 1/2 feet between poles usually works well. Set up 3 or four trot poles in front of the fence, and leave a space of 9' between the last pole and the fence. This space encourages the horse to bring the off hind up under itself and push off evenly with both hind legs.
Raised trot poles or cavaletti are for developing suspension and extension in the horse's trot; not really for jumping. (Cavaletti is often used to describe any sort of pole or rail; but technically, it should specifically refer to a series of raised poles.) Bounces or one strides can be constructed using raised poles or cavaletti, but that's another thing all together.
Honestly, you probably don't want or need to mess with trot poles or placement rails at this point. Setting the striding is tricky and takes some skill and practice, and if you don't have a ground person, requires a lot of getting on and off the horse.
But please do pay attention to ground lines, even when just larking over stuff in the pasture. It's easy to do and makes your horse's job easier.