So you know for a 2D polygon there are two types of symmetries, rotations and reflections.
Label your triangle's vertices counterclockwise
So firstly, how can you rotate the triangle? The rotation must take place about the centre of the triangle and after you have performed each rotation, the triangle must remain where it is.
So clearly a rotation of 0 degrees works. As you correctly stated, roations of 120 and 240 degrees work aswell (a rotation of 360 degrees is the same as a rotation of 0 degrees).
If you look to see what this is doing to your labelled vertices you will notice that a rotation of 120 degrees sends and
Now look for yourself how a rotation of 240 degrees acts on the letters.
Now for reflections. You correctly asserted that a line of symmetry must pass from the midpoint of an edge, through the centre and through a vertex. As there are 3 edges (and 3 vertices) there can only be 3 different reflections of the triangle.
Now observe what one of these would do to
Imagine reflecting in the line passing through the vertex A and the midpoint of BC, then , and
Again, repeat this for yourself with the other lines of reflection
Now you are finished you can observe that (3 rotations and 3 reflections)
It should also be clear now that your rotations and reflections are just reshuffling how you labelled your vertices so as you have demonstrated, the size of you group should be 3! = 6
Hope this helps, I can't think how to explain the group any more.