Are you going to store anything in the rafters???
I don't know your building codes over there, but on my side of town the span on the side between the 3 beams looks to be a bit too long for just a single board, even with a "L" angle iron added.
You have dead load and live load to calculate out, dead load would be the allowable dead weight on the side supports, this would be the total weight of the wood and any flooring, live load would be the allowable weight of anything put on top of the flooring.
How far is it between the 3 posts on the sides???