Pattern is flying to a set aerobatic schedule, against which you are judged. It is all about precision aerobatics. Plane is limited to 2meter wingspan, 2meter length and 5 kilos dry weight.

IMAC is also flying to a set aerobatic schedule, against which you are judged, but the plane has to be scale, ie based on a real full size aerobatic plane. There is no size or weight limit.

3D is all about freestyling, doing your own thing. Very few competitions as it is all about expression, and doing all the things that planes cant usually do.

The link that Zep gave above has lots of info on some of the popular 3D manouvers.