It would be better if the Venom spin-off is separate from the events of Spiderman 3. Tophar Grace said that he didn't want to do the Venom movie because he think it will be unsuccessful. It wouldn't be right if a completely different person portrays Venom in the spin-off movie yet it takes place right after the events of Spiderman 3 in which Topher was Venom. The only way that a different guy can portray Venom without causing any confusion would be if doesn't follow after Spiderman 3.