While drawing resonance structure of dibasic or tribasic acid, e.g. should I take out two $\ce{H+}$ (in dibasic) or three $\ce{H+}$ (in tribasic) together or one by while drawing resonance structure?
Do dibasic acids give both $\ce{H+}$ at once or one by one?
Actually, I was drawing resonance structure of $\ce{H3PO4},$ but got confused. Should I remove all three protons at once or only one?
It depends on what specifically the question is asking you to do.
In reality, a polyprotic acid such as $\ce{H3PO4}$ becomes deprotonated in a sequential way; the first proton is the easiest to remove (most acidic), then the second, then finally the third is hardest to remove (least acidic).
So there are four different species that the question might be asking you about: $\ce{H3PO4}$, $\ce{H2PO4-}$, $\ce{HPO4^2-}$, and $\ce{PO4^3-}$
Each of the three anionic species above has multiple resonance structures.
Resonance structures can only be drawn for single compounds. Rephrased: if a set of resonance structures are to be valid, then all atoms (nuclei) must be at exactly the same positions.
Thus, when asked to draw resonance structures of e.g. phenol or nitric acid, you cannot deprotonate or transfer protons. All protons must be exactly at the same spot in all structures, only electrons may move—thus, the rightmost structure of nitric acid drawn in scheme 1 is not a resonance structure. Distinct from the examples above, you can draw (a different set of) resonance structures for phenolate or nitrate after removing a single proton (see also scheme 1).
Scheme 1: possible resonance structures of (top to bottom) phenol, nitric acid, phenolate, nitrate. Lesser contributors to phenol and nitric acid in brackets. For nitric acid, an isomer that is not a resonance structure is shown.
Expanding on this, if you happen to have a diprotic or triprotic acid for which you can draw resonance structures such as carbonic acid or salicylic acid, your resonance structures must have the proton connected to the identical oxygen atom for the resonance structures to be correct, compare the examples in scheme 2.
Scheme 2: Resonance structures of carbonic acid and its conjugate bases (top three) and salicylic acid and its conjugate bases (bottom three). Further possible resonance structures of salicyldiate are omitted for brevity.
Finally note that your example is poorly chosen. Phosphoric acid and its conjugate bases only have one significant resonance structure each as depicted in scheme 3.
Scheme 3: Structures of phosphoric acid, dihydrogenphosphate, hydrogenphosphate and phosphate (from left to right). No other significant resonance contributors exist.
