Syntax Usage

Questions
1

I have two questions:

  1. How can I divide an entire list of numbers by three and only keep the remainder?
    I also want to keep track what number correlates to that new data set.

2)What other resources would I use for this kind of question?
I do not want to ask people and have to wait for answer every time a question arrises.

2

See the command QuotientRemainder—Wolfram Documentation for examples

3

example

data = RandomInteger[{1, 100}, {20}];
{data, Map[Mod[#, 3] &, data]}

or even

{data,Mod[data,3]}