Returns the Poisson distribution.
| Name | Type | Description |
| arg1 | number | The number of events. |
| arg2 | number | The expected numeric value, a positive number. |
| arg3 | boolean | A logical value (true or false) that determines the function form. If it is true, the function returns the cumulative Poisson probability. If it is false, the function returns the Poisson probability mass function. |