Why? Let’s see its Pros and Cons.
-It’s ability speed boost was super nice paired with sword dance
-Access to great moveset of Hi-jump kick, brave bird, and Fire blitz
-Could be a strong Psychical sweeper or Special Sweeper
-Gains a mega evolution which the other two don’t has one
-Defensive stat wasn’t too good, but still okay after Mega.
-Good Speed, Attack and Special attack
-Iron fist ability gives it a bonus stab power on Punch moves
-Could counter a lot of Pokemon in all the tier down Underused
-Medicore Defensive stat
-Could revange killed by the other easily
-No mega evolution
-Speed was still outspeed by the other Pokemon and easily KOed it
-Good HP stat
-Attack stat was also nice
-Special attack was nice
-Movepool was not bad
-Outclassed by the other two Pokemon you mentioned
-Bad Defense and Special defense, which means was still weak
-Speed of 65 was slow for no Trick room nor Trick room
-All the pros it has other two has
So this was why the main reason they were placed in different tier, Blaziken was Uber cuz it’s hidden ability and good movepool, Infernape was placed in UU cuz some other fire type and Emboar was placed in lower tier beacuase it’s Medicore speed and defensive stat.
Hope it helps!