(1) No.Resonance structures are ones with the same bonding sequence but different electronic configurations.In this case the two are not resonance structures,since the positions of hydrogen differ from each other.They are called tautomers.
(2) All are sp2 hybrid since the double bonds are involved.
(3) The triol tautomer is more stable.This is because the triol tautomer is aromatic.Although the trione form does have a aromatic resonance structure(in which the 3 nitrogen atoms bear 1 unit of +ve charge respectively and the 3 oxygen atoms bear 1 unit of -ve charge respectively),but the charges are too separated such that the resonance is of high energy and has small contribution to the real structure of the substance.
For Q3,please compare with the tautomerism of 2/4-hydroxypyridine & pyridine-2/4-one.In this case,the ketone tautomer predominates the mixture since the aromatic resonance of the ketone does not have a large separation of charges than the one in triazine,and that resonance does contribute much to the structure of the real substance.