If pure, why not trying an oven at 180 degree for a whole night. If the amount of water is too much and the mixture is a solution, then try distillation first.

Dry it in vacuo at 120 °C to constant weigh. However, you need not dry CrO3, because Pyridinium Chlorochromate (PCC) Corey-Suggs Reagent. Chlorochromic acid can by prepared by the dissolution of chromium trioxide in 6 M aq. hydrochloric ...

Thanks, Dr. Pazdera and Dr. Jondi for your kind suggestions.

Best regards.

CrO3 decomposes above 190ºC, so if you do dry it, use a much lower temperature.  Dr. Padera's conditions should be more than sufficient.  When making PCC, precise stoichiometry is not required, so weighing errors due to moisture are not an issue.