Yes you can put Venom in SM3. But a black suit movie pre-dating the Venom movie would be good and would allow depth.
I personally want Lizard in SM3 Raimi's last being SM4 with GG2 and a filler and black suit with new cast and director in SM5 and Venom in SM6 with the option of Venom and Carnage for SM7 to finish it.
But I think it WILL go GG2 in SM3 and black suit with Lizard and new cast and director for SM4 with Venom in SM6 and a conclusion with Venom and Carnage in SM6 with MJ being nothing more than a piece of ass after Raimi leaves ruining the credibility of these films.
But to cram the black suit an extra set-up villain Eddie's story Venom and if SM3 Harry's story (which Lizard offers a logical lead on until SM4 route while Venom does not) and MJ's story (relationship problems and marriage still has to be tackled) in SM3 is overstuffing one movie. Sure use imagination and I'm sure that is exactly what Stephen Sommers said when proposing the idea to Van Helsing "A 19th Century vampire/monster hunter who fights Jekyll/Hyde, Dracula, Frankenstein, The Wolf Man, Midgets from Hell, Vampire Babies, Vampire Hotties Babes, a love interest, the main character becomes a Wolf Man, Igor, a Q-rip-off who swears and has sex as a friar with modern weapons, and a cross between Indiana Jones and James Bond for the hunter" and when the studio execs look at him like "How?" I'm sure he said "C'mon use your imagination."
Just because you CAN do it DOESN'T mean you SHOULD do it. Get my drift?