Group 1 elements are comprised of: (1) hydrogen, sodium, lithium, potassium, rubidium, caesium, and francium. The tendency of these atoms is to lose one of their electrons such that they become charge +1.
Group 5 elements contains the vanadium, niobium, tantalum, and dubnium. It is quite hard to predict the oxidation number of these elements such that it will be hard to determine the number of atoms needed to react with them. Their valence electron is 5, and they react by losing them. Hence, in order to reach the octet rule, 3 group 1 elements should be reacted.