If your options (1 option = chance to buy 1 share, at a price that's typically fixed when the options was issued) vest over a three year period, it means you would be issued 1/(12*3) of your total options per month, for three years.
A common clause in such a vesting schedule is a "1 year cliff", meaning you don't actually get any options granted to you for the first year of employment. After a year, you get a third (1 of 3 years) worth of options in a lump sum, then the usual monthly amount each month after that.
If you fire employees before they reach their cliff, they don't get any options, and won't benefit at all from any liquidation event / sale / etc.
A common clause in such a vesting schedule is a "1 year cliff", meaning you don't actually get any options granted to you for the first year of employment. After a year, you get a third (1 of 3 years) worth of options in a lump sum, then the usual monthly amount each month after that.
If you fire employees before they reach their cliff, they don't get any options, and won't benefit at all from any liquidation event / sale / etc.