If you are a mystic, you will maybe answer that the makkos correspond to the sefiros, and the three sefiros of chabad that correspond to the final three makkos are qualitatively different than the other seven. Kol ha'kavod if you understand what that means.
The Ramban offers another answer. He suggests that the function of the last three makkos differed from the function of the earlier seven. The purpose of the first seven makkos was punishment: to force Pharoah and the Egyptians to admit that they were in the wrong. By the time makkas barad was over, that goal had been accomplished -- the Egyptians were ready to cry mercy. But that was not enough. The last three makkos were not a punishment, but were a demonstration of G-d's might, not only to prove G-d's power to the Egyptians, but also to stamp the memory of yetzi'at Mitzrayim on the psyche of Klal Yisrael for all generations. It's a subtle distinction. Why the first seven makkos were not enough to show G-d's might and give Klal Yisrael something to talk about and remember for generations to come is a question I can't answer.
The Abarbanel has a great answer. If you look at the reaction of Pharoah and the Egyptians to the earlier makkos, it's almost identical in every case. Moshe brings the makkah, Pharoah asks for the makkah to be removed, Moshe davens to Hashem and takes it away, and then Pharoah goes back to business as usual. If you look at the reaction to the threat of arbeh, something changes. Pharoah's servants beg him to do something before Egypt is destroyed, so Pharoah calls Moshe, listens to his demands, and instead of throwing him out, he asks, "Mi va'mi ha'holchim?" OK, who do you want to take with you? Let's negotiate. Maybe the men I can let go, but do you really need the women and children? Pharoah is at the bargaining table. Now, it's true that the negotiation fails and Pharoah doesn't give in, but the very fact that Pharaoh is at the table and talking is a dramatic change. From arbeh on, the geulah is a done deal - the rest is haggeling over price, so to speak. And so we read arbeh through the end as a separate parsha.
There is a mussar haskel here: once you are at the table to bargain, you've already lost the battle.
2) Last week I did a post on the reward given to the dogs for not barking during makkas bechoros. My wife pointed out that one can easily explain that the reward of "lakelev tashlichun oso" has nothing to do with whether animals deserve reward, but is an obligation upon us to show appreciation even to inanimate objects from which we have gotten benefit. This may explain the Mechilta, but the Yalkut is still difficult.
Why were the dogs given this reward of getting treifa meat? Ksav Sofer suggests that the dogs not barking served to distinguish Bnei Yisrael from the Egyptians. Not eating treifa is because "anshei kodesh tehiyun li," so that we may be distinguished by kedusha.