Tri-wing Baby Blender is not a good idea, and here's why:
1. As a rule of thumb, multi-wing planes should not have the distance between wings smaller than the wing chord (more precisely, a chord of the wider of the two wings) That's the first problem, because Baby Blender does have wide chord wings very close to each other and in fact already below that limit.If you add the third wing between the existing two, you will effectively add a lot of drag with no lift at all.
2. The airplane as "short-coupled" as Baby Blender will not "survive" adding more wing area without seriously lengthening the fuselage and/or making the stabilizers bigger. A lot.
3. Current BB design is already very sensitive to CG location. It doesn't seem to be good idea to add the third wing to the mix.
All the above indicates what to expect when doing such modification; but that doesn't mean it's not doable. I'm just afraid the airplane will effectively turn into big motor with tiny wings/control surfaces, and will no longer fly nice (if at all). To help with the problems mentioned above, your brother would need to make the wings longer and narrower, lengthen the fuselage and keep the tail light; bringing the plane closer to the silhouette of the Fokker Dr.1 (Dreidecker).