I think the answer is

Lacerated

Whole I am cut,

The definition of lacerate is to make deep cuts in

In parts I come in threes.

First I am scarlet,

lac is a resinous substance secreted as a protective covering by the lac insect usually red in colour.

Second I am first,

ace is often used to represent one in cards (also can be both high and low card).

Third I am an age,

era is a long an distinct period of time, an age.

Fourth I tell tales,

rat

Fifth I have eaten,

ate is the past tense of eat.

Sixth I scatter.

ted means to turn and spread out.

I think the answer is

Lacerated

Whole I am cut,

The definition of lacerate is to make deep cuts in.

In parts I come in threes.

The rest of the clues correspond to three letter words.

First I am scarlet,

lac is a resinous substance secreted as a protective covering by the lac insect usually red in colour.

Second I am first,

ace is often used to represent one in cards (also can be both high and low card).

Third I am an age,

era is a long an distinct period of time, an age.

Fourth I tell tales,

rat can be sland for a person who is deceitful or disloyal by telling tales.

Fifth I have eaten,

ate is the past tense of eat.

Sixth I scatter.

ted means to turn and spread out.