What is the Bronsted Acid of H3PO4 + HOH -------> H3O+ + H2PO4- ?

H₃PO₄ is a Bronsted acid as it is a proton donor. After donation of a proton (H⁺), H₃PO₄ becomes H₂PO₄⁻.

[On the other hand, H₂O (HOH) is Bronsted base as it is a proton acceptor. After accepting a proton, H₂O becomes H₃O⁺.]

A Bronsted acid can donate a proton ( essentially a H+ ion)
What has donated a proton in your equation
H3PO4 has gone to → H2PO4- It has lost a H+
HOH has gone to → H3O+ It has gained a H+

Now you can conclude - what has donated a proton and is therefore a Bronsted acid
And for completeness - what has accepted a proton and is therefore a Bronsted base?

how about its base?