These are the parameters for the antenna and the material of the horn antenna is 3 mm thick metal material. So is it possible to design a 30dbi gain at 3.3 GHz ?

 

Sure it is possible but it would be much more efficient to copy a successful design. There are not many people who could make an easy job of designing such an antenna.

 

Sure you can, horn antennas are easy to design - just flare out the waveguide slowly to whatever aperture you want. 30dBi at 3.3GHz corresponds to an effective area of 0.658 m^2, so if you make an allowance for the aperture efficiency from the expected aperture field distribution and flare outthe waveguide slowly to more than that you will be close. Copy the flare angles from dimensions of commercially available horns.

Making an optimum one, or getting a particular specified gain, needs more specialized help.