When it was first announced that there was going to actually be a third 'Hellboy' film, and that writer/director Guillermo del Toro would be returning at the helm, our first question was whether we would see Ron Perlman and Selma Blair reprise their roles as well.
As of now, the film is still listed as 'in development' on IMDB, and both Perlman and Blair are 'rumored' as returning for their roles. That means that the script hasn't been locked down, and a production date hasn't been set, so deals haven't yet been worked out for either Pelman or Blair to be reprising their roles.
There's also no hint as to what Hellboy 3 will be about, but you can bet that it will be as strong as the first two if del Toro is as involved. Guillermo del Toro is also extremely busy right now with 'Pacific Rim', which releases this summer. The official move-ahead with Hellboy 3 might very well depend on how well 'Pacific Rim' does, although it's doubtful that there's a direct correlation.
In addition to Perlman and Blair, who are obviously the main characters, other actors that could potentially return for Hellboy 3 are Doug Jones [also rumored] and Seth MacFarlane [rumored].