I'd like to see Seagal in EX3 (and I'm a huge Stallone fan) But I won't lose sleep if he doesn't feature in it.
The Expendable movies are good (but not great) I much prefer Sly (and the other cast) doing their own 'solo' stuff (Bullet To The Head' was awesome.....but no-one went to see it)
EX2 under-performed at the box-office. And as good as it was to see Van Damme back on the big screen....it's not exactly put JCVD back on the map, has it? The problem being that (apart from Sly, and maybe Statham?) there's 'too-little' to spread amongst the other expendables...Randy Couture must have around 3 or 4 lines of dialogue, likewise with Terry Crews (I think Jet Li realised this, and took a back-seat in EX2) Personally, I think the Expendables should only star the older (school) actors like Stallone, Schwarzenegger, Seagal, Willis, Norris, etc (and dump Statham, Couture, Crews, etc)
And the only worrying problem with these 'Ensemble' movies, is that they damage the chance for any solo movies (look at the box-office for 'The Last Stand' & 'Bullet To The Head' as an example) Now you can blame lousy marketing/popularity/trends/downloads/etc to a certain extent, but logic dictates that there's little reason to see (or rent) a solo project from one of the Expendables cast....when you can get a dozen for the price of one (come the next Expendable movie)
Even the DTV market is pairing up it's stars at the moment. So it's obvious that the Expendable franchise (however fun) have set a trend, in which one big star will no longer sell a cinema OR a dvd release anymore?
So as much as I'd like to see Seagal act with Sly (on the big screen) above and beyond the one movie, it'll do little for further their careers at this point.