A sine wave gets it's peaks clipped, and the more you turn it up, the more it turns into a square wave - the smooth single harmonic of a sine wave gets additional harmonics as the waveform gets sharper.
Yes, dbFS is a term for digital audio levels, relative to the 0dB maximum point a fixed-point number can be represented in a fixed-point audio file.
Yes, but you can only have +3dBFS if the audio signal can be represented by the numbers - ie, if you had a 24-bit wave file, there is no signal in that that can be at +3dBFS, because the maximum level that can be represented is 0dBFS, period.
However, in Logic's mixer, before it's output to the world where it's converted back into fixed-point audio, audio signals are represented as 32- or 64-bit floating numbers, which *can* go higher than 0dBFS. A lot higher in fact - you can turn signals up to, say, +1500dBFS, and they won't get distorted (as long as you turn them back down before you hit the fixed point output, of course.)