Complex sentence: Senna was not the only young driver to run near the front-Stefan Bellof finished 3rd (and was gaining on both Senna and Prost) but the Tyrrell team's results were erased later in the season due to weight infringements.
Split into simpler sentences: Senna was not the only young driver to run near the front-Stefan Bellof finished third and had been closing on both Senna and Prost. The Tyrrell team's results were erased later in the season due to weight infringements.