The star of the series, Idris Elba, revealed in an interview with the UK's Guardian, earlier this year, that season 3 would debut in September.
Given the tweet above from BBC One, the channel that airs the series in the UK, it looks like it'll return this summer, which begins not-so-long from now, given that we're in the middle of May.
And like last season, I assume the BBC will make the episodes of next season available here in the USA within 24 hours or so after they air in the UK. That seems to be the trend, with other UK series that have USA followings, making episodes available to USA audiences immediately after they are broadcast in the UK - like Misfits did last year.
And as for what we can expect in season 3, Elba previously revealed that, first, expect 2 or 3 new characters to be introduced, although he won't say more, other than one of the new characters, a baddie most likely, will be a huge challenge for Luther (he kind of dismissed the previous Luther baddies as blips, suggesting that this new baddie, to be introduced in season 3, will be his most formidable to date; we can only wonder what this character's make-up will be - Luther's Professor Moriarty, likely just as mental, except he/she will use his/her *powers* for evil),
Second, don't expect the Alice story line to return in any sort of significant way (he's said that she'll still be in the Luther universe, but suggested that she wouldn't be of much influence on anything or anyone; he reiterated that they're trying to set things up for the "bigger picture" and there's a possibility that Alice's story strands come back if/when the feature film is made; but don't expect to see much of her in season 3).
And lastly, an interesting item he added is that, Luther will be closer to retirement than he’s ever been - a story line that could bleed into the feature film's narrative. I can see it; Luther's last case - a case for the ages, with his greatest foe ever - before he hangs up the hat; the challenge of the case, and the baddie in season 3 push him over the edge, and essentially force him into retirement.
Warren Brown, Nikki Amuka-Bird, Dermot Crowley and Michael Smiley all return to assume the roles they each played last season.
Season 3 guest stars will include Orlessa Altass, Lucian Msamati (Game Of Thrones fans will remember him as the pirate Salladhor Saan; he and Nonso Anozie were the 2 black Brits who joined the series cast last season), and Ned Dennehy (Tyrannosaur).