What was important about the discovery of gallium, scandium and germanium for the acceptance of mendeleev's Theory about the organization of the elements?

They filled in the gaps in his Periodic Table and confirmed its usefulness.

Explanation:

Mendeleev's Periodic Table of 1869 contained gaps.

He argued that the gaps represented elements that were not yet discovered.

He gave these elements provisional names and used his table to predict their properties.

When the elements were later discovered in the 1870s, their properties matched his predictions remarkably well.

The confirmation of his predictions convinced chemists that the properties of elements varied periodically with their atomic masses (later changed to atomic number).