Now I have 3 builders, and I want to add one more. Is that possible without charge? If so, please tell me how to do that.

Yes.
To add a builder, you need to buy one from the shop.
Look under the resources option.
You will need to buy builders using gems.
Adding a forth builder will cost you 1,000 gems.
You can only have up to 5 builders.
The fifth and the last builder will be for 2,000 gems.
To get gems without any charge, you need to complete more tasks which you can find under your achievements record.
You will get gems for completing tasks.
You can also collect gems from obstructions that you take such as trees, trunks and sometimes a gem box containing 25 gems.