WHICH ONE IS ACIDIC..m-aminophenol, m-methoxyphenol, phenol.?

Phenol. The others will give more electron density on oxygen by involving in resonance, thus decreasing the acidic nature. 

from the meta position only inductive effect can operate.

there can be no resonance from the substituents attached at the meta position even if they had lone pairs or double bonds.

the amine and methoxy groups are electron donating via resonance only if they are at the ortho or para positions.

but they are also electron withdrawing via negative inductive effect as the nitrogen and oxygen atoms are very electronegative.

if substituents are capable of exhibiting both resonance and inductive effect then it depends on their relative position that which tendency will dominate. at the ortho and para positions resonance will dominate and at the meta position inductive effect will dominate.

it is known that electron withdrawing effects stabillise the conjugate base by dispersing the negative charge density and hence increase the acidic nature and vice versa.

therefore out of the given compounds m-methoxyphenol is the most acidic as oxygen is more electronegative than nitrogen.