explanation says that divide them into 3 groups 5-5-6 and if first two are equal, eliminate them.

I see the point, but why don’t we divide them into 7-7-2. So if first two are equal, eliminate, and only compare the last with 2 blocks to get the result. In this case the least number of weightings will be 2.

Why not?

What if it does turn out that one of the “group of 7” contains the heavier block?