How did Mendeleev predict 3 elements?

When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium, eka-silicon, and eka-manganese, with respective atomic masses of 44, 68, 72, and 100.

When did Mendeleev predict elements?

In 1871 Mendeleev predicted the existence of yet undiscovered element he named eka-aluminum (because of its proximity to aluminum in the periodic table).