In Chicken Invaders 3, instead of the key grinding madness present in later games, the unlockables here are obtained through completing the mission.

There are 7 unlockables, and the range of unlocks depends on the difficulty of the mission.

This difficulty only has 3 unlockables in its unlock pool. The first complete playthrough will unlock "Secret Weapon", the second will have "Super Secret Mine Bombs" and the third - "Damage Indicator". There are no further unlockables from playing on this difficulty.

This difficulty has 3 new unlockables in its unlock pool. Beating Veteran also counts as beating Rookie at the same time, so you may get one unlock from Veteran's pool and one from Rookie's pool. In terms of Veteran's unlock pool, "Superstar Hero" is first, "Progress Indicator" is second and "Points Indicator" is third.

Beating Superstar Hero for the first time will give you "Clupea Harengus", and it has no other unlockables in its own pool. Beating it also counts as beating Rookie and Veteran at the same time, so you will get one Rookie and one Veteran unlock.

Here's a table that sums it up:

Given that Rookie and Veteran only have 3 unlocks each while Superstar Hero only has one, you only need to complete the game a total of 3 times to unlock everything.

The first playthrough must be on Veteran as Superstar Hero is only made available after that, then you need to play Superstar Hero once and Veteran once more.

The order can be either "Veteran, Superstar Hero, Veteran" or "Veteran, Veteran, Superstar Hero".

Depending on how fast you are at completing missions, this will only take between 3 and 5 hours.