The periodic table is arranged in order of increasing atomic number as you go across a period. It is also arranged according to the number of electrons that will fit in a given energy level. So period 1 is also energy level one, with only two electrons. Period 2 is energy level 2 with a maximum of 8 electrons. The same is true for period 3,with room for 8 additional electrons. Because there can only be up to 8 more electrons in the third energy level, there is a maximum of 8 elements in period 3. Once you get to element 19, the additional electrons move into the 4th energy level - period 4.
Do you mean group 3??
If so, they are placed in groups due to their valency (number of electrons in the outer most shell of the atom), their reactivity levels, similar properties.
Sorry that is all I can think of.