<p>I want to draw kossel delta corner in fusion 360 for 2040 aluminium extrusion like on picture below, but cant find a way to actualy start, I draw 3 side polygon and 20x40mm rectangle but cant go from there, so do you have any suggestion?</p>

<p><a href="https://i.stack.imgur.com/hxAR4.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hxAR4.jpg" alt="enter image description here"></a></p>

I attempted to create your drawing but discovered that an important set of parameters is missing. You have to have either the intersection point of the legs (73.34) from each side or the angle between the legs (73.34) and the base (106.41) to create construction lines. Once you have either of those items, you can construct the remainder of the design using offsets, radii, etc.
More accurately, one other missing item that would be required to complete this design is the placement of the holes at the top (12) relative to some other feature of the design.
Having taken on the challenge of your drawing, I've found that it is necessary to surrender. The angles or the intersection point are critical and without them, no solution comes to my alleged mind.
I have also discovered one additional datum missing. The distance of the bottom truss and the thickness of this truss would be required to provide a more certain solution.
One the flip side, I've found
alpha-tech3d.com
which appears to include similar parts, rotated 180° with what appears to have all of the necessary data.