THREE RHYMING RIDDLES

1)
In the beginning, no one will object
That I am the matter, the issue, the subject.
Change my third letter and then I become
Poisonous, hazardous, even fatal to some.
Change it again, and then you'll find
I'm just the thing for your body and mind.

2)
My first is in gumball but not in glamour,
My second in both saw and hammer;
My third begins start while my fourth begins end,
My fifth and sixth are in brand but not trend;
My seventh and eighth, which are both the same letter,
Are found in retell, but not in better.

3)
Sometimes it glitters, but often not;
It may be cold, it may be hot.
Ever changing, though the eye can't measure,
Concealed within may be great treasure.
Some find safety beneath its gate,
While some may die beneath its weight.

Answers:

1. TOPIC/TOXIC/TONIC
2. BASEBALL
3. ROCK