Decimal fields in tables

[fio productions]

Hi,
I have a database table which has 3 decimal(8,2) fields, 'price', 'tax'and 'total'. they receive their values,
from input fields in a form.

The decimals are all messed up when I put a number like 100.000 .
I understand that in order for me to have 3 floats I had to create the
db field type something like 'decimal(8,3)' ?

My question is: How to make a decimal field accept any number of floats, if it's '10.45' or '100.000' ?

Thanks in advance,
Mike