Is SO3 ionic or covalent

SO3 is COVALENT. It is sulphur trioxide. and is used to make sulphuric acid.

At STP it is a gas , and is dissolved in more sulphuric acid to make oleum (fuming sulphuric acid) ( H2S2O7). Oleum is 'NASTY' stuff. Oleum is then siddolved into water to form sulphuric acid.

SO3 + H2SO4 = H2S2O7

H2S2O7 + H2O = 2H2SO4.