Introduction:
In order to determine the relative acid strength of the given compounds, we need to consider various factors such as the number of ionizable hydrogen atoms, the electronegativity and size of the central atom, and the stability of the resulting conjugate base.

Explanation:

1. H3PO4 (Phosphoric Acid):
- Phosphoric acid has three ionizable hydrogen atoms (H+) which can contribute to acidity.
- The central atom, phosphorus, is electronegative and relatively large, which makes it easy for the hydrogen atoms to dissociate.
- The resulting conjugate base, H2PO4-, is stable due to the resonance stabilization provided by the phosphate group.
- Phosphoric acid is a moderately strong acid.

2. H4P2O7 (Pyrophosphoric Acid):
- Pyrophosphoric acid has four ionizable hydrogen atoms (H+).
- The central atom, phosphorus, is still electronegative but now it is bonded to another phosphorus atom, which increases the size of the molecule.
- The resulting conjugate base, H3P2O7-, is less stable than H2PO4- due to the absence of resonance stabilization.
- Pyrophosphoric acid is stronger than phosphoric acid.

3. H5P3O10 (Metaphosphoric Acid):
- Metaphosphoric acid has five ionizable hydrogen atoms (H+).
- The central atom, phosphorus, is still electronegative and now it is bonded to two other phosphorus atoms, making the molecule larger.
- The resulting conjugate base, H4P3O10-, is less stable than both H3P2O7- and H2PO4- due to the absence of resonance stabilization.
- Metaphosphoric acid is the strongest acid among the given compounds.

Increasing Order of Acid Strength:

Based on the explanations above, we can arrange the given compounds in increasing order of acid strength as follows:

1. H3PO4 (Phosphoric Acid)
2. H4P2O7 (Pyrophosphoric Acid)
3. H5P3O10 (Metaphosphoric Acid)

Summary:

- Phosphoric acid (H3PO4) is the weakest acid among the given compounds due to the presence of resonance stabilization in its conjugate base (H2PO4-).
- Pyrophosphoric acid (H4P2O7) is stronger than phosphoric acid because its conjugate base (H3P2O7-) lacks resonance stabilization.
- Metaphosphoric acid (H5P3O10) is the strongest acid among the given compounds as its conjugate base (H4P3O10-) does not have any resonance stabilization.