@TDC - Tyrion's battle plan failing is just the usual head-fake we get from GOT; when things are going well expect a couple dozen spanners in the works.
@Dragoncat - Daenerys' problem is a lack of dragonriders; she can't deploy them separately as the dragons only listen to her, and even so she can't use them too often because she has to be fighting personally, and she herself is much more vulnerable.
I expected the Highgarden army to fail as they are said in the books to be the weakest fighters in Westeros despite being the most numerous, but didn't expect TPK :D I wanted Jaime to break and run Olenna through after she taunted him, only for her to smirk at him as if to say "gotcha!"
The issue of the 3-headed dragon is going to be resolved differently in the TV show I think. We only have 2 candidates in the show, Daenerys and Jon, with zero setup for the Tyrion angle in the previous 6 seasons. In the books there's young Griff aka Aegon who's doing well in the snippets we've seen, plus who knows who else GRRM might introduce.
The 3-headed dragon prophecy might also mean that Daenerys is the first dragonrider ever to control 3 dragons at once, leaving out any other Targaryens...
...or we'll all be completely fooled once again, let's see :D