As with the pre-production buildup to Clerks 2 it seems a lot is going to depend on whether or not Jeff Anderson (Randal) signs on to the project to reprise his role one more time. If you're familiar with the background to the last Askewniverse film you know that Anderson was a longtime holdout to making #2 because he didn't think it would be well received, would be a pale shadow of the original film and might even cause typecasting damage to his own career if he was seen as being "that Clerks guy who does the funny walk and always shows up in Kevin's movies." Jeff held out for a good while and was the last major actor to agree to appear, and while he ended up loving #2 and thinking that the story was brilliant and funny he seems to have retreated back into his bubble of "meh, is this really a good idea?"
In a recent SModcast podcast related to Clerks 3 Smith said Anderson's involvement was going to be important if not crucial, but in the meantime he's going to stick to plans to make the movie and see what happens.
Actually, I think his issue with doing III has more to do with money issues stemming from II and failure by the studio to pay out back end money. There was a Hollywood Bable On where Kevin went into some detail on how he had to hire a laywer to audit the weinstine company, and they were owed something like ten pulse million in unpaid royalties. Kevin, Jay, Jeff, and Brian all took basically no pay up front, shifting to a backend deal... And the company screwed them. Jeff has said he won't do the third till the company pays him the owed money from clerks II.