Forrest has a lot of flaws. A good heart, but circumstances have enabled him to become a bit overrated. If Makh does stop Forrest (and I think your 3 round prediction is a good one) - it will definitely help Makh's reputation. So this is a good fight - even if I don't think Forrest is a step up from Wach. Makh just needs a promoter with deeper pockets; hopefully this fight helps him land one.
I don't think he looked good vs Zhang or Pulev. Zhang seemed dead after the fourth, and it was clear there was something wrong. But vs Franklin and Hunter, he performed better than any of your non-prospect 40 to 30 ranked culprits would have. He's solidly top 30 material. I also have to defend Washington, he had well above average power. He KOd Helenius, Forrest and kept Wilder honest with his power for some rounds. You have to remember that he started boxing at 29, debuted at 30 and had 0 amateur fights, so he didn't have much ability to set up his shots vs skilled opponents.
Forrest seems to be taking over from Hammer, and Rudenko as far being a tough roadwarrior HW fringe contender goes. Takam and Sokolowski are still "any time, anywhere" types who usually still give a good account of themselves even if they've seen better days. I don't most boxing fans appreciate fighters like these guys enough.
That is also true of Forrest, but I don't expect Forrest to win or make it very far in the HW division. I expect Makhmudov to win this fight, but he has to get better at throwing the jab and set up shots or he will get exposed the moment he steps up.
I never said Takam himself should be the fight, I'm just giving an example of the sort of level Makh should step up to being nearly 34 years old, and probably approaching the end of his prime. The fact that Forrest may be his best opponent is ridiculous, considering how many people highly rate Makh