I know, this answer is almost three years late, but the existing answers leave important issues open.

Now make it a triplane, which reduces the chord to roughly 1/3, increasing the aspect ratio from 6 to 18, just like a glider. That's a 300% increase in efficiency, no?

No. Reducing the chord will do exactly nothing to induced drag. With the same span and the same mass, induced drag remains unaffected. However, by vertically spacing the wings generously, a slight reduction of induced drag will be possible because more air is involved in lift creation.

But only if mass remains unchanged.

Instead of a heavy spar for a thin wing, you could just put the stabilator at the wing tips joining the 3 wings: so no extra wing strut interference drag ( expect for a small amount of extra interference drag at 2 wings roots).

Again, no. If you have no shear transmission between the single wings, each has to carry its lift just like any cantilever wing. With 1/3 the chord and 1/3 the lift per wing, this will require three massive spars because each spar cap will be three times larger than one of a scaled down cantilever wing.

Bi- and triplane wings are lighter for two reasons:

1. Lower maximum speed of the whole airplane, so the maximum dynamic pressure for which the wings need to be sized is much smaller, and
2. Bracing allows the lower wing to act as the tension member and the upper wing as the compression member of a truss. But that requires shear transfer between the two, which is traditionally done by bracing.

Without bracing, the triplane wing will become much heavier than the monoplane wing of the same span. Joining the tips of the three wings will not change this - at the wingtip is no bending moment, so there is no benefit in transferring loads between the tips.

Bracing will only help when done along the span, not at the tip alone. And the viscous drag of bracing will require a much larger and heavier engine to keep this triplane in the air. Sure, now the braced wing is lighter, but those gains will be more than eaten up by the heavier engine and higher fuel load for a meaningful range.

I know, this answer is almost three years late, but the existing answers leave important issues open.

Now make it a triplane, which reduces the chord to roughly 1/3, increasing the aspect ratio from 6 to 18, just like a glider. That's a 300% increase in efficiency, no?

No. Reducing the chord will do exactly nothing to induced drag. With the same span and the same mass, induced drag remains unaffected. However, by vertically spacing the wings generously, a slight reduction of induced drag will be possible because more air is involved in lift creation.

But only if mass remains unchanged.

Instead of a heavy spar for a thin wing, you could just put the stabilator at the wing tips joining the 3 wings: so no extra wing strut interference drag ( expect for a small amount of extra interference drag at 2 wings roots).

Again, no. If you have no shear transmission between the single wings, each has to carry its lift just like any cantilever wing. With 1/3 the chord and 1/3 the lift per wing, this will require three massive spars because each spar cap will be three times larger than one of a scaled down cantilever wing.

Bi- and triplane wings are lighter for two reasons:

1. Lower maximum speed of the whole airplane, so the maximum dynamic pressure for which the wings need to be sized is much smaller, and
2. Bracing allows the lower wing to act as the tension member and the upper wing as the compression member of a truss. But that requires shear transfer between the two, which is traditionally done by bracing.

Without bracing, the triplane wing will become much heavier than the monoplane wing of the same span. Joining the tips of the three wings will not change this - at the wingtip is no bending moment, so there is no benefit in transferring loads between the tips.

Bracing will only help when done along the span, not at the tip alone. And the viscous drag of bracing will require a much larger and heavier engine to keep this triplane in the air. Sure, now the braced wing is lighter, but those gains will be more than eaten up by the heavier engine and higher fuel load for a meaningful range.