In my opinion Venom is likely to at least make an appearance in #3. This is because even though venom may not be the longest running villain in the Spiderman comics, he is probably the most widely known bey people who are only familiar with the comic books in passing. He has all the right ingredients for being in a movie:
-evil (always a plus in villains)
-an alien, which makes him all the more sinister to us humans for the simple fact that it plays on the dark nature of humans which makes us prone to hate/fear all things that are foreign
-has many of the powers that spiderman has which makes him sinister by being the perversion of something which is accepted to be good and benevolent
-has the ability to disguise itself/himself which adds to the suspense because he/it could potentially be anywhere
-fairly one dimensional, with clear cut goals ...... kill spiderman, hurt humans, ..... which makes it easy to incorporate the character into the movie where time is extremely restrictive. It is much easier to incorporate a shallow character into a movie and develop him/her further in the movie as needed, rather than trying to introduce a complex character which will inevitably need to trimmed down due to time constraints.
Having said all that, I really would be surprised, and somewhat disappointed if the Hobgoblin was not in #3. The storyline so far seemed like it was progressing towards that, with the death of Norman Osbourne and the fact that Harry thinks that Spiderman is the bad guy, and pretty much the entire second movie.
I also think that if they do bring Venom in to the third movie they will make it something like the cartoon serries in the nineties. They can not possibly fit the Secret Wars into the trilogy. If I remember correctly they already introduced Jameson's son the astronaut, who is the one who brings Venom to earth in the cartoon.