I've used Emboar and Infernape, but not Blaziken. As for which is my favorite...
I like all fairly the same amount (which isn't to say much since all three are the least favorites of their respective trios for me) but if I had to choose one I'd say Blaziken, which is kinda funny considering it's one of the few more popular Pokemon whose popularity in the fandom perplexes me; I don't think it has a lot of traits that generally make a Pokemon popular--or maybe that's just my own bias. But I digress. I prefer Blaziken over the other two as I think it looks the coolest out of the three--Infernape just doesn't sit right with me while Emboar is a bit too... dumpy in my opinion, for a lack of better words, to edge out Blaziken. If I had to rank them:
Easily Emboar. I love the fact that he defies a lot of both Fire and Fighting-type design tropes with his 'ugly' face and hulking gait. His stat distribution and movepool are also both really interesting; a lot of species with high HP stats but low defenses are often pretty poor, but Emboar only needs to tank enough hits to make the foe very, very fainted. One of my favourite starters, not to mention Fire-types.
Never used him competitively, but always my immediate Gen V playthrough pick.
Blaziken. Speed Boost turned it into a Mon-star. Although I'm perfectly fine with the other two, to be honest. Considering I'm bias to Fire starters most of the time, it only makes sense that I like all three pretty much equally. However, I liked Blaziken's design ever since the Johto era anime and wanted Ash to get one (sadly, he didn't, obviously), and I find Torchic to be absolutely adorable. That said, really, all three are pretty much equals to me, as they each fulfill different roles despite sharing the same typing, so they all have their own charm and flare (see what I did there, heh). But, of the three, Blaziken will always be my favorite of them, but only by a really small amount.
I love them all (just not too much into Blaziken's mega evolved form), but I have to go with Infernape which also is overall my favourite Fire-type Pokémon of all time. Those colourful traits appearing on its figure, which almost makes me think of a shaman, look amazing and don't even let me start to talk about how perfect of an Iron Fist's ability user my beloved infernal ape is.