Will a C16 deck joist (about 2" by 5") span 3m (about 15 feet)?
Are you asking whether a 2x5" joist will span 15 feet (3m)? No. It'll be a trampoline.  (Or yes, but it'll be a trampoline.) Here are your options as I see them:


Go with your original plan and lay it on the soil. Then you'll have critical intermediate support even with loose soil. Worst-case scenario it settles into the soil and ends up lower then it started, or becomes uneven in the case of non-uniform soil. 
Lay a sleeper at the midpoint to cut the joist span in half. In this case I'd remove 4-6" of soil under each of the three sleepers and replace with crushed rock.
(Preferred method): Remove 4-6" of soil and replace it with crushed rock laid over heavy landscape fabric. Then set your joists right on the rock.