it's free and / free of charge / for free What is the difference?

Its all the same, possibly you might see 'free of charge' in a business or similar situation where other items and services have a specific charge.

For free

It's free / free of charge / for free

There is no difference. All tree mean that a certain object or service does not cost any money.

But the first one can be used in another conext as well. For example: The bird was caged but now it's free.

I hope this helps.