I moved your question to the physical sciences forum -- getting ready for the science fair is more about communicating your results after the experiment is done, so more experts will see your question here.
It will probably help you to read some discussion of similar experiments as well as the project guide, particularly the parts about variables:http://www.sciencebuddies.org/science-f ... ndex.shtmlhttp://www.sciencebuddies.org/science-f ... p039.shtmlhttp://www.sciencebuddies.org/science-f ... =26&t=2086
In short, it's only fair to compare different stain removers on the same kind of stain (and with all other conditions/variables kept constant, for example the kind of fabric used, amount of stain applied and time allowed to set, and method of application of the stain remover). So if you only did 3 tests, it would be better to try removing stain X with vinegar, then X with lemon, then X with seltzer. You wouldn't be able to draw any conclusions if you tried to remove X with vinegar, Y with lemon, and Z with seltzer, because you'd have changed more than one variable. You might consider repeating each of the three tests a few times (multiple "trials") to see how much variation there is.
If you have more time, it could be interesting to repeat all of the above with different kinds of stains. So if you had stains X, Y, and Z and 3 different stain removers, there would be 9 experiments times a few trials of each. Due to the chemistry involved, some stain removers would probably work better or worse on different kinds of stains. However, that level of explanation/investigation might be a bit much if your child is in elementary school -- it's more important to have fun and get some really basic experience with the scientific method.