An LLC protects you from liability, but for contract development work there's likely no need to set one up as you aren't typically exposed to much liability risk in that context. For tax purposes, LLC doesn't really mean anything to the IRS. If you have a single-member LLC, it will be taxed as a sole-proprietorship, or you can elect to have it taxed as an S-Corp or C-Corp.
The advantage of the S-Corp is that you can avoid some self-employment tax by paying yourself a fair wage with some of the income and distributing the remaining earnings to yourself as non-wage income. The reasonable wage part is subjective and people who try to take advantage of it can be challenged by the IRS.
Currently, there is a 20% pass-through deduction that makes S-Corp less attractive than it formerly was for many people. For a 3-month engagement in the current tax environment I wouldn't bother with an LLC/S-Corp election.
Be sure to do some research on which expenses you can rightfully use to offset your business revenue.