Truth Table: Logic & Reasoning

Hi there. I am hoping someone can at least point me in the right direction.

How would I begin to go about composing a truth table to find out the logic (or lack thereof) of this statement:

If the following are true: a is necessary for x, b is necessary for x and c is necessary for x.

How would you compose a table for statement saying that a and b are necessary for x, but c is not necessary for x?

I know this is probably not very clear and just hope this even remotely makes sense to someone!

TIA!

Jen