H3PO4 is also called phosphoric acid. It is a strong mineral acid, meaning that under the right conditions it readily dissociates into two separate ions: a proton (H+) and the conjugate base (H2PO4)-. This is depicted in the chemical equation below:
H3PO4 --> (H+) + (H2PO4)-
A strong acid readily undergoes this dissociation to form free protons under the right conditions. This means that the phosphoric acid has to be in a medium that stabilizes the charged species produced in the chemical equation. Since "like dissolves like", a highly polar medium is required to promote the dissociation of strong acids. Water is a highly polar medium, so strong acids like phosphoric acid readily dissociate to form protons in water. Concentrated sulfuric acid (H2SO4) is 98% pure, meaning it only contains 2% water content. This means that the phosphoric acid is less prone to dissociate in the sulfuric acid than in the pure water.
All of this means that phosphoric acid in water is more acidic than phosphoric acid in sulfuric acid.