...A 3 element Yagi-Uda offers less than 1 dB over a 2 element.
Well I suppose that depends on the specific design you choose. I've got a 3-element yagi
design with 9.3 dBi free space gain, and haven't found a practical 2-element that does better
than about 6.8 dBi.
But regardless of the exact difference, using fewer elements isn't always bad, especially if
the original element spacing was compromised to accommodate multiple bands. I used just
two elements of a tri-band yagi for a while with the full boom length because in that case
the original spacing was shorter than optimum on 20m (to allow it to work on 10m.)
Depending on the shape of the attic, if 2 elements allows greater spacing from ducting and
such, it may be a better choice than more elements.
But try just the driven element first to see how it works.