Have anyone a plan how to bend large sircles with 30mm solid round iron.It must be 3 metres in diametre ?

3 meters in dia is about 18 feet or a full stick of stock.

You can use things like a forked tree as a bending fork or use a couple of pieces of pipe welded to something heavy they are solid. 1-1/4 inch is thick stuff so you may want to heat it so it bends easier. Build the fire next to the bending fork and go a little bit at a time, always checking against the pattern (a circle in the dirt) to be sure you are at size.

You could build a 45 or 90 degree section of the curve and use that as a template and bending jib to bend against. Hot will not spring back much, but check against the pattern early and often.

When you get to the ends, weld an extension onto the stock for leverage, then cut it off when finished. You may be able to slip a piece of pipe over the round bar for leverage.

I am coming up with 9.4 meter of material, or 30.8 ft. Do you get stock in those lengths? I suspect that making 3 or 4 sections so you can cut the excess off and weld the whole thing together will work better than trying to make this in one go, but that also is affected by what equipment you have.

Phil