I have thoroughly enjoyed this show its been a wonderful good quality series and so much better to watch than reality TV or never ending game shows. I do hope they go another season at least..... if not a couple more seasons.
Now that the casts' three year contracts are up - apparently - most of the main cast would need to be willing to commit to a fourth series for it to happen, it's not just the BBC's decision this time. If even one of the four Musketeers ( actors ) wants to move on with their career, series 4 won't happen, unless a reduced series would tempt them back.
There's also the fact that, in order for the show to remain interesting, changes to the dynamics of the Musketeers would need to happen - eg Athos can't remain Captain indefinately, otherwise the other three can never move forward within the ranks, although like others I think Aramis will have a seperate future to d'Artagnan and Porthos. I personally wouldn't want a fourth series unless relationships and dynamics started to change.
In real life, Louis died young, leaving Anne to take over until her son was old enough to become King. That's the kind of major changes I'd want to see in a fourth series.
I wouldn't want the series to go on so long it gets stale, but I would like to see a Season 4 and 5, after that they should asses if they can keep it fresh, and if they can't the show needs to be put to rest.
I wouldn't be surprised if they all went into it knowing they would only commit to 3 seasons. It's crazy how fast it went by. I hope Santiago Cabrera and Howard Charles work together again, at least.