The ability to change the configuration of the pump discharge nozzle will, of course, depend solely on the design of the pump. If it is not supported below the pump casing, it may be possible to rotate the casing to direct the nozzle in a horizontal configuration. This should not cause any problem with the hydraulic operation of the pump. Naturally if there are support feet under the casing, you will not be able to do that without losing the support.

The discharge line should always be at least one size larger than the discharge nozzle size on the pump, therefore your 3 inch discharge line is not too big for the discharge nozzle size you indicate.