80x2 beams are in the homebuilding plans because these are universally availbable the world over while having a good ration between weight and stiffness.

Not all stiffness comes from the beams, the stiffness of the beams landings and torsional stiffness of the hulls between the beams is important as well. Basically, we need to find out first what the case of your problem is and adress that.

Going for 80x3 mm beams will give you 50% more beam stiffness but this will not translate directly into 50% stiffness increase of the whole platform because of the other factors named above.

Weight increase (for 2 beams) will be about 4 kg.

It is worth a try, but I don't expect more then 25% stifness increase.

How much does the boat flex now ?

Measuring this involved laying the platform up on its sterns (without mast etc) and seeing how far one bow needs to be lifted before the other bow is lifted clear of its support as well.

By the way the nacra 500 (and nacra 5.0) used the beams of the larger boats like the Nacra 6.0 and F18's and that makes them very stiff for the Nacra 500. Of course this comes at a cost, in the case of the Nacra 500 this cost is roughly 50 kg additional boat weight and 20% slower performance.

Stiffness of a platform is important but certainly not all important. More stiffness is mostly better in the way of feel in very choppy conditions. In the way of all-out speed I'm not sure how important is really is.

Wouter