An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+). In other words, an acid increases the concentration of H+ ions in an aqueous solution. This protonation of water yields the hydronium ion (H3O+); in modern times, H+ is used as a shorthand for H3O+ because it is now known that a bare proton (H+) does not exist as a free species in aqueous solution.
Sodium/natrium is a metal from first column group so it should have one 1+ charge. Phosphate ion has 3- charge. That is why there 3 natrium ion for 1 phosphate ion when this molecule is dissolved in water. The ion formula would be:
(Na)

(PO

) ==> 3 Na

+ PO
True because it seems that is like a pattern it's always 6