If you're thinking of watching 'The Expendables 3,' then 'part 3' is a kind of odd place to start any franchise. Therefore, I would generally recommend starting with part 1 and going from there. However, in case you're in any wonder as to what it's all about, it's basically an ensemble cast (led by Sly Stallone) of all the biggest (and best?) action stars of the eighties (plus Jason Statham thrown in for good measure).
What you have is a bunch of old men who really should be playing golf and yet we suspend our disbelief long enough to believe that they can really run, jump, punch and generally take on wave after wave of insignificant bad guys without so much as getting shot. So, if you generally like (or can at least remember) some of these faces from yesteryear and the epic on-screen battles they fought then you'll probably enjoy the nostalgia trip which this film basically is. It never takes itself seriously, so don't expect anything dark and moody, just fun.
The first Expendables outing was definitely fun. And, for the record, I enjoyed part 2 even more. Therefore, I was really expecting to LOVE the third instalment. Don't get me wrong, I enjoyed it, but there was one thing I didn't approve of. Namely the fact to interject some 'fresh blood' into Sly's team. Basically, it wasn't necessary. Sly chooses youngsters over the established faces who we actually care about. Therefore, we don't really know them as well as the older action stars and these youngsters take up a reasonable proportion of the first third of the film. Then they end up getting captured and the old team has to be assembled to get them back. Basically, they could have left out the whole 'youngsters subplot' and got on with our aging heroes hunting down rogue expendable baddie, played by Mel Gibson.
I think if I was going to watch this again (and I probably will!), I'll skip the first third with the 'kids' and go straight on to the rescue mission. Plus I thought there was too much CGI explosions for my liking – but I'm no fan of computer added trickery, unless it's really seamlessly intertwined with the film. Half the time you could tell the explosions were added post production.
Anyway, if you like the first two films, or any of the stars, you should get something out of this. Just don't be too misled by the advertising posters – not all the stars featured play a major part. Some are only in the film for literally a few minutes.