Was the original subfloor one piece or three cut pieces? Any time you create a joint, you'll diminish the integrity, somewhat. The question should be, "will I compromise the flooring beyond it's function?". If you can place an additional piece under the joints it will add strength. If the adhesive you use to attach the 1/4" overlay bonds both pieces and the adhesive is stronger than the wood plys (it will be), then the total strength will be similar to the original. Remember to observe direction of plies when using plywood as a subfloor.